daemon: Replace root_mounted global with intelligence.

daemon: Replace root_mounted global with intelligence.

We used to maintain a global flag 'root_mounted' which tells us if the
user has mounted something on root (ie. on the sysroot directory).

This flag caused a lot of trouble (eg. RHBZ#599503) because it's hard
to keep the flag updated correctly when the user can do arbitrary
mounts and also use mkmountpoint.

Remove this flag and replace it with a test to see if something is
mounted on *or under* the sysroot.  (It has to be *or under* because
of mkmountpoint and friends).

This also replaces a rather convoluted "have we mounted root yet"
check in the mount* APIs with a simpler check to see if the mountpoint
exists and is an ordinary directory.