Implement INSTALL_PLUGINDIR build flag that only applies to MariaDB Connector C / MariaDB client

Currently libmariadb3.install packaging for Debian in upstream MariaDB looks like this:

usr/lib/*/libmariadb.so.*

usr/lib/mysql/plugin/client_ed25519.so

usr/lib/mysql/plugin/dialog.so

usr/lib/mysql/plugin/mysql_clear_password.so

usr/lib/mysql/plugin/sha256_password.so

usr/lib/mysql/plugin/client_ed25519.so

This has two problems:

• plugins paths are not multi-archified
• plugin paths are not versioned

In Debian official repos this is fixed like this:

usr/lib/*/libmariadb.so.*

usr/lib/*/mariadb19/plugin/client_ed25519.so

usr/lib/*/mariadb19/plugin/dialog.so

usr/lib/*/mariadb19/plugin/mysql_clear_password.so

This is however not an ideal fix.

First of all the path name mariadb19 is inherited from the server INSTALL_PLUGINDIR. This is set in debian/rules for the entire server+client build. The server binaries end up in a versioned path which isn't necessary. There should be a separate INSTALL_PLUGINDIR_CLIENT or something.

Secondly, the mariadb19 reflects the server ABI "version". For client stuff is should reflect the client version, so a better name would be for example mariadb3, libmariadb3 or mariadbclient3.

Implementing something like INSTALL_PLUGINDIR_CLIENT would allow to set this.

Eventually I would like to have this libmariadb3.install:

usr/lib/*/libmariadb.so.*

usr/lib/*/mariadb3/client_ed25519.so

usr/lib/*/mariadb3/dialog.so

usr/lib/*/mariadb3/mysql_clear_password.so

And one day, somewhere far far away, in libmariadb4.install:

usr/lib/*/libmariadb.so.*

usr/lib/*/mariadb4/client_ed25519.so

usr/lib/*/mariadb4/dialog.so

usr/lib/*/mariadb4/mysql_clear_password.so