layout-dependent my.cnf location

Currently mariadbd (and all clients) search for my.cnf in hardcoded locations /etc and /etc/mysql, except for *BSD where they use /usr/local/etc.

This doesn't work well on Ubuntu, that uses /etc/mariadb and for binary tarballs that want to have local config, independent from other mariadb installations.

Let's do this:

• there is already a concept of "install layout" that sets various paths for deb/rpm/tgz differently, let's use that
• rpm uses /etc as before
• deb uses /etc/mariadb as Ubuntu prefers, and /etc/mariadb/my.cnf ends with a line ?include /etc/mysql/my.cnf so if anyone has configs in there — they'll still work.
• tgz uses etc/ in the basedir, and etc/my.cnf ends with ?include /etc/my.cnf

in the last two cases we preserve the backward compatible behavior, while allowing the user to opt-out of it (by deleting the ?include line).