Create an 'overlay' property (default 'off') that will control wether
the filesystem should be mounted even if the mountpoint is busy or if
it should fail with a 'mountpoint not empty'.

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