Also make initial_database optional target on non-Windows, as sometimes
it can be quite handy to bootstrap without MTR.

Parser: uninitialized Lex->create_last_non_select_table under
mysql_unpack_partition() fix.

Tested with main, parts suites.

Declare innodb_purge_threads as 4-byte integer (UINT)
instead of 4-or-8-byte (ULONG) and adjust the documentation string.

Fixing a test failure introduced by MDEV-21743

partially reverting 961413d2

incomplete. locking issues

because it is not

in particular, after checking for duplicates in a partitioned table,
do not re-check it for every partition individually

This patch removes support for LinuxThreads.
It was superseded by NPTL in Linux 2.6 (2003).