This patch includes:
- MDEV-19639 sql_mode=ORACLE: Wrong SHOW PROCEDURE output for sysvar:=expr
- MDEV-19640 Wrong SHOW PROCEDURE output for SET GLOBAL sysvar1=expr, sysvar2=expr
- Preparatory refactoring for MySQL WL#4179

Detailed change list:

1. Changing sp_create_assignment_lex() to accept the position
   in the exact query buffer instead of a "bool no_lookahead".
   This actually fixes MDEV-19639.
   In the previous reduction sp_create_assignment_lex() was
   called too late, when the parser went far the from beginning
   of the statement, so only a part of the statement got into
   sp_instr_stmt.

2. Generating "SET" or "SET GLOBAL" inside sp_create_assignment_instr()
   depending on the option type.
   This fixes MDEV-19640.
   In the previous reduction the code passed (through no_lookahead)
   the position of the
   word GLOBAL inside sp_create_assignment_lex(), which
   worked only for the left-most assignment.

3. Fixing the affected rules to use:
   - ident_cli instead of ident
   - ident_cli_set_usual_case instead of ident_set_usual_case

4. Changing the input parameter in:
   - LEX::set_system_variable()
   - LEX::call_statement_start()
   - LEX::set_variable()
   from just LEX_CSTRING to Lex_ident_sys_st for stricter data type constrol:
   to make sure that noone passes an ident_cli
   (a fragment of the original query in the client character set)
   instead of server-side identifier
   (utf8 identifier allocated on THD when needed).

5. Adding Lex_ident_sys() in places where the affected functions are called.

6. Moving all calls of sp_create_assignment_lex() to the places
   just before parsing set_expr_or_default.
   This makes the grammar clearer, because
   sp_create_assignment_lex() and sp_create_assignment_instr()
   now stay near each other, so the balance of LEX's push/pop
   can be read easier.
   This will also help to WL#4179.

7. Adding class sp_lex_set_var
   Moving the initialization code from
   sp_create_assignment_lex() to the constructor of sp_lex_set_var.
   This will also help to WL#4179.

8. Moving a part of the "set" grammar rule into a separate
   rule "set_param".
   This makes the grammar easier to read and removes
   one shift/reduce conflict.

client-certkey.pem contains both certificate and corresponding
private key.

This removes dependency of libhmash

A thread_pool_stall_limit which is smaller than default would result
in quicker creation of threads.

row_search_mvcc(): Duplicate the logic of btr_pcur_move_to_next()
so that an infinite loop can be avoided when advancing to the next
page fails due to a corrupted page.

Also, --skip-innodb-buffer-pool-load-at-startup to avoid a crash
in buf_load() due to loading pages that we are corrupting intentionally.

Apprently, sometimes there will be null pointers with 0 length
passed to the MyCTX::update() function, and  will need to return
a valid buffer.

So weaken the assertion, and use a valid pointer for src if it was NULL.

At higher levels of innodb_force_recovery, the InnoDB transaction
subsystem will not be set up at all.

At slightly lower levels, recovered transactions will not be rolled back,
and DDL operations could hang due to locks being held at all.

Let us consistently refuse all writes if the predicate
high_level_read_only holds. We failed to refuse DROP TABLE
and DROP DATABASE. (Refusing DROP TABLE is a partial backport
from MDEV-19570 in the 10.5 branch.)

- Replace mysql_version check with omit_virtual_cols() in
ha_innobase::check_if_supported_inplace_alter().

- If one of the encryption threads already started the initialization
of the tablespace then don't remove the other uninitialized tablespace
from the rotation list.

- If there is a change in innodb_encrypt_tables then
don't remove the processed tablespace from rotation list.

LOCK_global_system_variables: Declare with MYSQL_PLUGIN_IMPORT

Changed the function append_range_all_keyparts to use sel_arg_range_seq_init / sel_arg_range_seq_next to produce ranges.
Also adjusted to print format for the ranges, now the ranges are printed as:
    (keypart1_min, keypart2_min,..)  OP (keypart1_name,keypart2_name, ..) OP (keypart1_max,keypart2_max, ..)

Also added more tests for range and index merge access for optimizer trace

- Don't apply redo log for the corrupted page when innodb_force_recovery > 0.
- Allow the table to be dropped when index root page is
corrupted when innodb_force_recovery > 0.

The update callback functions for several settable global InnoDB variables
are acquiring InnoDB latches while holding LOCK_global_system_variables.

On the other hand, some InnoDB code is invoking THDVAR() while holding
InnoDB latches. An example of this is thd_lock_wait_timeout() that is
called by lock_rec_enqueue_waiting(). In some cases, the
intern_sys_var_ptr() that is invoked by THDVAR() may acquire
LOCK_global_system_variables, via sync_dynamic_session_variables().

In lock_rec_enqueue_waiting(), we really must be holding some InnoDB
latch while invoking THDVAR(). This implies that
LOCK_global_system_variables must conceptually reside below any InnoDB
latch in the latching order. That in turns implies that the various
update callback functions must release LOCK_global_system_variables
before acquiring any InnoDB mutexes or rw-locks, and reacquire
LOCK_global_system_variables later. The validate functions are being
invoked while not holding LOCK_global_system_variables and thus they
do not need any changes.

The following statements are affected by this:

SET GLOBAL innodb_adaptive_hash_index = …;
SET GLOBAL innodb_cmp_per_index_enabled = 1;
SET GLOBAL innodb_old_blocks_pct = …;
SET GLOBAL innodb_fil_make_page_dirty_debug = …; -- debug builds only
SET GLOBAL innodb_buffer_pool_evict = uncompressed; -- debug builds only
SET GLOBAL innodb_purge_run_now = 1; -- debug builds only
SET GLOBAL innodb_purge_stop_now = 1; -- debug builds only
SET GLOBAL innodb_log_checkpoint_now = 1; -- debug builds only
SET GLOBAL innodb_buf_flush_list_now = 1; -- debug builds only
SET GLOBAL innodb_buffer_pool_dump_now = 1;
SET GLOBAL innodb_buffer_pool_load_now = 1;
SET GLOBAL innodb_buffer_pool_load_abort = 1;
SET GLOBAL innodb_status_output = …;
SET GLOBAL innodb_status_output_locks = …;
SET GLOBAL innodb_encryption_threads = …;
SET GLOBAL innodb_encryption_rotate_key_age = …;
SET GLOBAL innodb_encryption_rotation_iops = …;
SET GLOBAL innodb_encrypt_tables = …;
SET GLOBAL innodb_disallow_writes = …;

buf_LRU_old_ratio_update(): Correct the return type.

Remove the separate allocation and pointer indirection of
dict_v_col_t::v_indexes.

C++11 defines the singly-linked std::forward_list. Prefer it to
the doubly-linked std::list in cases where we dot really need it.
Also, clean up some code.

dict_index_remove_from_v_col_list(): Remove.
Obsoleted by dict_index_t::detach_columns().

There is no std::forward_list::push_back(). Use push_front() instead.
The ordering does not really matter.

dict_v_col_t::n_v_indexes: Added. There is no std::forward_list::size(),
and trx_undo_log_v_idx() needs to know the size.

rtr_info_track_t::rtr_active: Encapsulate. There really was no justification
for the pointer indirection.