Improved management of container

This pull request makes the container more manageable:

 * `/var/lib/mysql` will be initialized if this is empty on startup. This makes it easy to use directory mounts without having to worry about copying mysqls data files.

 * If `/var/lib/mysql` has been initialized (as per above), `/var/lib/mysql/setup` is scanned for SQL scripts that will automatically be run before the container completes the boot proces and comes online.

 * The container now shuts down mysqld upon exit to avoid corruption of data files. Inside the container, `sv stop mysql` and `sv start mysql` now works as expected.

 * The `setup.sh` idea seems to be defunct for several reasons, so I removed it.

https://github.com/mzedeler/docker-mysql to be more specific.