Create an 'overlay' property

This (default 'off') will control wether the filesystem should be mounted even if the mountpoint is busy or if <code>zfs</code> should fail with a 'mountpoint not empty' as usual.

Doing overlay mounts is the default mount behaviour on Linux, but not in ZFS. It have been decided that following the ZFS behaviour should be the default, but this overlay allows for site administrator to override this decision.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Closes: #2503, #2442