Post-fix: add comments, remove dead-code.

Variant #4 of the fix.

Make ORDER BY optimization functions take into account multiple
equalities. This is done in several places:
- remove_const() checks whether we can sort the first table in the
  join, or we need to put rows into temp.table and then sort.
- test_if_order_by_key() checks whether there are indexes that
  can be used to produce the required ordering
- make_unireg_sortorder() constructs sort criteria for filesort.

When simplify_joins() converts an outer join to an inner, it should
reset the value of TABLE::dep_tables.  This is needed, because the
function may have already set TABLE::dep_tables according to the outer
join dependency.

don't use MY_THREAD_SPECIFIC on the client side
in embedded:
1. server memory accounting does not track client
   side memory
2. in not embedded builds, this still *might* be
   useful for plugins (handlersocket, connect, etc)

including the test case:

https://github.com/mysql/mysql-server/commit/520aedfe
INNODB: "DATA DIRECTORY" OPTION OF CREATE TABLE FAILS WITH PWRITE() OS
ERROR 22
Fix for version mysql-5.6
PROBLEM
========
For version mysql-5.6.27 onwards InnoDB fails to create a table
with explicit 'data directory' option when Innodb_flush_method
is set to O_DIRECT.While creating link file we get a  a pwrite
error 22 due to the alignment restrictions imposed by O_DIRECT
flag  which is being set for the link file created.

FIX
===
Fixed the above issue by making use of file IO functions while
creating the link file that wouldn't let the O_DIRECT flag
restrictions arise.
Reviewed-by: Kevin Lewis <kevin.lewis@oracle.com>
Reviewed-by: Shaohua Wang <shaohua.wang@oracle.com>
RB: 11387

Do not allow NULL password to pass directly to password
validation plugin.

An additional warning saying "tc-log cannot be enabled"
is emitted when InnoDB is installed at runtime on mysqld
built with wsrep-patch (-DWITH_WSREP=ON).

This happens because, installing InnoDB increments the
total number of 2pc-capable engines and with wsrep-patch
already enabled - the total count goes >1. Even though,
this condition is sufficient to enable tc-logging, it is
not permitted at runtime, and thus the warning.

Updated the testcase to avoid the warning.

Ha_trx_info should not be reset in ha_fake_trx_id() as it is
later used during commit.

This is to ensure that the list of valid values for
METADATA_LOCK_INFO fields is always kept in sync with
the parent MDL types.

Xtrabackup-based SST method uses socat. Move it from
Suggests to Depends list.

10.1 introduced a problem:
Execution time for various recursive stages
(walk, update_used_table, and propagate_equal_fields)
in NULLIF is O(recursion_level^2), because complexity is
doubled on every recursion level when we copy args[0] to args[2].

This change fixes to avoid unnecessary recursion in:
- Item_func_nullif::walk
- Item_func_nullif::update_used_tables
- Item_func_nullif::propagate_equal_fields
when possible.

         CREATE function.

         Test case added.

take MDL_SHARED_WRITE instead of MDL_SHARED_NO_READ_WRITE
for OPTIMIZE TABLE. For engines that need a stronger
lock (like MyISAM), reopen the table with
MDL_SHARED_NO_READ_WRITE.

Two bugs here:
* the server could create an frm (with a long attribute
  value) that it could not read back
* Connect engine opened files from inside DROP TABLE
  and was ignoring the error (correctly) but was not
  hiding it from the server (incorrectly). This caused
  a crash later when DROP TABLE was finishing successfully
  while stmt_da already have seen an error.

Also added a text case for
  MDEV-7935 CREATE TABLE ... AS SELECT ... can cause a Server crash (Assertion `0' in Protocol::end_statement)
because Connect stopped clearing the error status
in stmt_da as a fix for MDEV-7935

* my_yyabort_error() helper
* s/lex->thd/thd/
* remove 'else' after MYSQL_YYABORT (for consistency,
  95% of the parser did not use 'else' in this case)
* simplify ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE

when reading table options

install private generated files

Do not mark subquery as inexpensive when it is not optimized.

        Compiler there is strict about the C/C++ call model
        mixing in function variable assumptions.
        Fixed by adding some 'extern "C"' and changing
        '?' operator with 'if'.

MySQL 5.6 do not work with MariaDB 10.1

Analysis: Problem is that tablespace flags bit DATA_DIR
is on different position on MySQL 5.6 compared to
MariaDB 10.1.

Fix: If we detect that there is difference between dictionary
flags and tablespace flags we remove DATA_DIR flag and compare
again. Remote tablespace is tried to locate even in case
when DATA_DIR flag is not set.

otherwise the plugin does not get compiled if
cmake is called multiple times.

        The check_user_can_set_role() used find_user_exact() to get the
        permissions for the SET ROLE NONE command. Which returned NULL too often,
        for instance when user authenticated as 'user'@'%'.
        Now we use find_user_wild() instead.

Added mysql_to_mariadb.sql script, to change mysql.user tables from
MySQL 5.7 to MariaDB.
After this script is run, one can get the other tables fixed by running
mysql_upgrade