Move actually mounting the root filesystem into its own series of hooks.

Move actually mounting the root filesystem into its own series of hooks.

We now have mount hooks.  They are sourced in an infinite loop until one of
them actually mounts the real root filesystem.

This makes it easier to add support for arbitrarily complex schemes to find
the root filesystem without having to patch the init script.

This patch series is also avaialble ass the hookify-finding-root branch at
http://git.fnordovax.org/dracut