hammer2 - Allow simple 'mount @label <target>' shortcut for snapshots
* If any hammer2 PFS on a device is already mounted, all other PFS's on
the device can be mounted simply by specifying their label. There is
no need to specify the device. e.g.:
# hammer2 pfs-list /build
Type ClusterId (pfs_clid) Label
MASTER
726d8ab1-9839-11e7-98a7-
6145cb9ac050 ROOT
MASTER
726d8a72-9839-11e7-98a7-
6145cb9ac050 LOCAL
SNAPSHOT
eb19b5fa-98a7-11e7-98a7-
6145cb9ac050 ROOT.
20170913.102102
#
# mount @ROOT.
20170913.102102 /mnt