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

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It was superseded by NPTL in Linux 2.6 (2003).

Problem:
=======
When we upgrade from "mysql" to "mariadb" if slave is using repositories as
tables their data is completely ignored and no warning is issued in error log.

Fix:
===
"mysql_upgrade" test should check for the presence of data in
"mysql.slave_master_info" and "mysql.slave_relay_log_info" tables. When tables
have some data the upgrade script should report a warning which hints users
that the data in repository tables will be ignored.

This task deals with packing the sort key inside the sort buffer, which  would
lead to efficient usage of the memory allocated for the sort buffer.

The changes brought by this feature are
  1) Sort buffers would have sort keys of variable length
  2) The format for sort keys inside the sort buffer would look like
     |<sort_length><null_byte><key_part1><null_byte><key_part2>.......|
      sort_length is the extra bytes that are required to store the variable
      length of a sort key.
  3) When packing of sort key is done we store the ORIGINAL VALUES inside
     the sort buffer and not the STRXFRM form (mem-comparable sort keys).
  4) Special comparison function packed_keys_comparison() is introduced
     to compare 2 sort keys.

This patch also contains contributions from Sergei Petrunia.