The extension of the record comparison functions for SPATIAL INDEX in
mysql/mysql-server@b66ad511b61fffe75c58d0a607cdb837c6e6c821
was suboptimal for multiple reasons:

Some functions used unnecessary temporary variables of the int type,
instead of the more appropriate size_t, causing type mismatch.

Many functions unnecessarily required rec_get_offsets() to be
computed, or a parameter for length, although the size of the
minimum bounding rectangle (MBR) is hard-coded as
SPDIMS * 2 * sizeof(double), or 32 bytes.

In InnoDB SPATIAL INDEX records, there always is a 32-byte key
followed by either a 4-byte child page number or the PRIMARY KEY value.

The length parameters were not properly validated.
The function cmp_geometry_field() was making an incorrect attempt
at checking that the lengths are at least sizeof(double) (8 bytes),
even though the function is accessing up to 32 bytes in both MBR.

Functions that are called from only one compilation unit are defined
in another compilation unit, making the code harder to follow and
potentially slower to execute.

cmp_dtuple_rec_with_gis(): FIXME: Correct the debug assertion
and possibly the function TABLE_SHARE::init_from_binary_frm_image()
or related code, which causes an unexpected length of
DATA_MBR_LEN + 2 bytes to be passed to this function.

Currently, if a user wants to require TLS for every connection made
over the network, then every user account on the system needs to be
created with "REQUIRE SSL" or one of the other TLS options.

Implementing a require_secure_transport system varuable (which,
in particular, can be set using the --require_secure_transport=ON
command line option) in the MariaDB Server would make it a lot
easier to require TLS (or other secure transport) system-wide.

This patch implements this new system variable, adds the ability
to set it with SQL statements, from the command line and from the
configuration file, and also contains improvements for mtr that allow
the user to establish non-secure TCP/IP connections (for example,
to verify the operation of the new option).

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).