Set new debian runtime dir

[lvoytek]

Similar to #5059, it would be nice to move away from /run/mysqld to /run/mariadb as the directory to contain MariaDB's socket and pid files.

I noticed the duckdb code also prefers /run/mysqld, and would be happy to include the swap for that here too if that seems reasonable.

[ottok]

This suggestion makes sense to me and I am in favor of merging this on main and having it in the next MariaDB release. Considering various other things that have already been renamed mysql -> mariadb doing this too is logical and I don't foresee any regressions with it, as /run is anyway a transient path.

[gkodinov]

Thank you for your contribution! This is a preliminary review.

FWIW, "mysqld" refers to the MySQL Daemon (mysqld binary). The equivalent of that for MariaDB is mariadbd. Not mariadb.

Also, please consider merging the 4 commits with a message compliant to coding_standards.md.

And also merging the whole thing into the extended MDEV-40964 as suggested.

[lvoytek]

Reduced to 1 commit and moved to /run/mariadbd. The folder makes sense to me, I just swapped it to /run/mariadb originally to match the RPM setting INSTALL_RUNDATADIR_RPM. I'll copy this over to #5059