vgextend: Allow PVs to be added to VGs that have PVs missing

Allowing people to add devices to a VG that has PVs missing helps
people avoid the inability to repair RAID LVs in certain cases.
For example, if a user creates a RAID 4/5/6 LV using all of the
available devices in a VG, there will be no spare devices to
repair the LV with if a device should fail.  Further, because the
VG is missing a device, new devices cannot be added to allow the
repair.  If 'vgreduce --removemissing' were attempted, the
"MISSING" PV could not be removed without also destroying the RAID
LV.

Allowing vgextend to operate solves the circular dependency.

When the PV is added by a vgextend operation, the sequence number is
incremented and the 'MISSING' flag is put on the PVs which are missing.

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