[Toybox] Promoted mount.c.

[Rob Landley]

That took a long time to debug.

The result's still got various loose ends (a bunch of TODO comments,
several LSB options not implemented yet, it needs a test suite entry, I
have yet to test user mounts), but it works well enough to swap in to
aboriginal linux and build stuff. It's implementing bind mounts,
loopback mounts, and move mounts. (The infrastructure's there for mount
-a and remount but I haven't tried them yet...)

(At some point, I should open the can of worms that is shared subtrees...)

Good enough to cut a release with. On to the rest of the todo list
(ping.c and the useradd/del/group stuff).

Well, dinner first. :)

Rob