Initial MariaDB entity

Largely a clone of the existing mySQL entity. Drop-in replacement for MySQL at runtime, compatible with all scripts and config options -- even the binaries are still called `mysql-*`.

Still TODO:
* Only tested against default mirror (UK-hosted `mirrors.coreix.net`); need to verify URLs for others. Seems that some mirrors take you through intermediate pages (e.g. project donation) before starting the download proper.
* No pre-built OS/X artifacts, so currently throws `UnsupportedOperationException` on non-Linux platforms, including `localhost` on Macs