This flagSet type allows the user to also set the value of the retain
snapshots flag, something unique to subvolume removal (all others had
the force flag only).
Signed-off-by: John Mulligan <jmulligan@redhat.com>
To prepare for multiple flags types, we create a flagSet interface and
break out the functionality that was in Update into a per-type method
and then the common "merger" function.
Signed-off-by: John Mulligan <jmulligan@redhat.com>