LVM-activate: failover with missing PVs

There area two changes included:

• Allow the system ID to be changed on a VG when the VG is missing PVs, as long as a majority of PVs are still present. This requires a recent version of lvm that supports the --majoritypvs option for vgchange.

• Use --activationmode degraded when activating LVs so that raid LVs can be activated when legs are missing, as long as sufficient devices are available for raid to provide all the data in the LV.

By David Teigland.