vgchange -aay: improve unexpected command variations

vgchange -aay: improve unexpected command variations

For completeness and consistency, adjust the behavior
for some variations of:

  vgchange -aay --autoactivation event [vgname]

The current standard use is with a VG name arg, and the
command is only called when all pvs_online files exist.
This is the optimal case, in which only pvs_online devs
are read.  This remains the same.

Clean up behaviors for some other unexpected uses of the
command:

. With no VG name arg, the command activates any VGs
  that are complete according to pvs_online.  If no
  pvs_online files exist, it does nothing.

. If a VG name is used but no PVs online files exist for
  the VG, or the PVs online files are incomplete, then
  consider there could be a problem with the pvs_online
  files, and fall back to a full label scan prior to
  attempting the activation.