Some I/O functions and macros that are declared in os0file.h used to
return a Boolean status code (nonzero on success). In MySQL 5.7, they
were changed to return dberr_t instead. Alas, in MariaDB Server 10.2,
some uses of functions were not adjusted to the changed return value.

Until MDEV-19231, the valid values of dberr_t were always nonzero.
This means that some code that was incorrectly checking for a zero
return value from the functions would never detect a failure.

After MDEV-19231, some tests for ALTER ONLINE TABLE would fail with
cmake -DPLUGIN_PERFSCHEMA=NO. It turned out that the wrappers
pfs_os_file_read_no_error_handling_int_fd_func() and
pfs_os_file_write_int_fd_func() were wrongly returning
bool instead of dberr_t. Also the callers of these functions were
wrongly expecting bool (nonzero on success) instead of dberr_t.

This mistake had been made when the addition of these functions was
merged from MySQL 5.6.36 and 5.7.18 into MariaDB Server 10.2.7.

This fix also reverts commit 40becbc3
which attempted to work around the problem.

Make it visible

https://github.com/MariaDB/server/pull/777#issuecomment-496469366

option values now have to be edited in-place,
instead of replacing an option with another one
that has the same name but a new value.

- --disable-plugin-load or --skip-plugin-load will now reset plugin load
  list. Needed for future changes to mtr tests

- multiple usage of plugin-load-add and optimizer-switch are now included
  in generated config files

Problem:
=======
fil_iterate() writes imported tablespace page0 as it is to discarded
tablespace. Space id wasn't even changed. While opening the tablespace,
tablespace fails with space id mismatch error.

Fix:
====
fil_iterate() copies the page0 with discarded space id to imported
tablespace.

fix MDEV-18750: failed to flashback large-size binlog file

fix mysqlbinlog flashback failure caused by reading io_cache without MY_FULL_IO flag

fix MDEV-18750: mysqlbinlog flashback failure on large binlog

- Use corrupt page id instead of whole block after releasing it from
LRU list.

MDEV-19600: The optimizer should be able to produce rows=1 estimate for unique index with NULLable columns

Modify best_access_path() to produce rows=1 estimate for null-rejecting
lookups on unique NULL keys.

MDEV-18015: Assertion `global_status_var.global_memory_used == 0' failed when using UDF , window functions and views

Adding destructor for Group_bound_tracker to free Cached_item_str.
The Cached_item for window functions are allocated on THD:mem_root
but the Cached_item_str has value of type string which is allocated on
the heap, so we need to call free() for it

os_file_write_func() and os_file_read_no_error_handling_func() returned
different result values depending on if UNIV_PFS_IO was defined or not.

Other things:
- Added some comments about return values for some functions

Wrong compiler warning from GCC:
‘snprintf’ output 2 or more bytes (assuming 4001)
into a destination of size 4000

- --default-character-set can now be disabled in mysqldump
- --skip-resolve can be be disabled in mysqld
- mysql_client_test now resets global variables it changes
- mtr couldn't handle [mysqldump] in config files (wrong regexp used)

Do not use TARGET_LINK_LIBRARIES(mysys mysys_ssl),
this means that mysys_ssl is linked to practically everything.

The issue is that two MARIA_HA instances shares the same MARIA_STATUS_INFO
object during UNION execution, so the second MARIA_HA instance state pointer
MARIA_HA::state points to the MARIA_HA::state_save of the first MARIA instance.

This happens in
thr_multi_lock(...) {
...
	for (first_lock=data, pos= data+1 ; pos < end ; pos++)
	{
...
		if (pos[0]->lock == pos[-1]->lock && pos[0]->lock->copy_status)
			(pos[0]->lock->copy_status)((*pos)->status_param,
																	(*first_lock)->status_param);
...
	}
...
}

Usually the state is restored from ha_maria::external_lock(...):

\#0  _ma_update_status (param=0x6290000e6270) at ./storage/maria/ma_state.c:309
\#1  0x00005555577ccb15 in _ma_update_status_with_lock (info=0x6290000e6270) at ./storage/maria/ma_state.c:361
\#2  0x00005555577c7dcc in maria_lock_database (info=0x6290000e6270, lock_type=2) at ./storage/maria/ma_locking.c:66
\#3  0x0000555557802ccd in ha_maria::external_lock (this=0x61d0001b1308, thd=0x62a000048270, lock_type=2) at ./storage/maria/ha_maria.cc:2727

But _ma_update_status() does not take into account the case when
MARIA_HA::status points to the MARIA_HA::state_save of the other MARIA_HA
instance.

The fix is to restore MARIA_HA::state in ha_maria::external_lock() after
maria_lock_database() call for transactional tables.

Removed #! sequence from non-executable script.

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.

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