Make share unmount operation a necessary condition in remove_share. Provide a
mount suffix as an optional argument to check mounts.
Bugfix: umount nfs export mount when deleting the last export of a given share.
Don't unmount the share when sharing via samba is turned off even if there is
no active sharing/exporting going on for the share.
Don't allow share deletion when there are active snapshots, nfs exports or
samba shares for the given share. When these constraints are satisfied, clean
umount the share and only when successful, delete the share.