mariadb: refresh plugins

mariadb: refresh plugins

This commit deals with changes related to plugins and how they're built
or disabled.

Currently a lot of plugins are packaged which are merely for tests or
plain examples. Other distros do not bundle these, hence this commit
does away with them.

A few new plugins are added related to PAM and Kerberos (auth_gssapi,
auth_gssapi_client and auth_pam).

The BuildPlugin template is refactored to also allow building of library
plugins (needed for auth_gssapi_client). The template is also cleaned up
- some extraneous dollar signs are removed and the install function is
now defined outside the template.

Unwanted plugins/engines are now turned off efficiently (without using
cmake variables) by blanking CMakeLists.txt files in the associated
folders. The idea was lifted from Gentoo.

ha_sequence is now built into the server. This is an upstream
preference. The plugin is about 30 kbytes in size, so there is no harm
adding it into the server, which weighs in at about 15 Mbytes anyway.

Last but not least the auth_socket plugin is now also built into the
server.  This allows the local root user to login to the database
without a password being set. This makes maintenance easier without
being detrimental to security. The idea was lifted from Debian.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>