Make the 'zpool import' command honor the overlay property to allow
filesystems to be mounted on a non-empty directory. As it stands now
this property is only checked by the 'zfs mount' command. Move the
check into 'zfs_mount()` in libzpool so the property is honored for all
callers.
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#3227
9540be9b23