Add 'fail_if_percent_unsupported' arg to _percent() and _percent_run().

Add 'fail_if_percent_unsupported' arg to _percent() and _percent_run().

We unfortunately don't yet _know_, in dev_manager_snapshot_percent(), if
a snapshot-merge target is active (activation is deferred if dev is
open); so we can't short-circuit origin devices based purely on existing
LVM LV attributes.

Set 'fail_if_percent_unsupported' in dev_manager_snapshot_percent() for
a merging origin LV, otherwise passing unsupported LV types to _percent
will lead to a default successful return with percent_range as
PERCENT_100.

For a merging origin, PERCENT_100 will result in a polldaemon that runs
infinitely (because completion is PERCENT_0).