pv: check for the PV_EXT_USED flag and deny pvcreate/pvchange/pvremove/vgcreate on...

pv: check for the PV_EXT_USED flag and deny pvcreate/pvchange/pvremove/vgcreate on such PV (unless forced)

Make sure we won't use a PV that is already marked as used. Normally,
VG metadata would stop us from doing that, but we can run into a
situation where such metadata is missing because PVs with MDAs
are missing and the PVs left are the ones with 0 MDAs.

(/dev/sda in this example has 0 MDAs and it belongs to a VG,
but other PVs with MDA are missing)

$ pvs -o pv_name,pv_mda_count /dev/sda
  PV         #PMda
  /dev/sda       0

$ pvcreate /dev/sda
  PV '/dev/sda' is marked as belonging to a VG but its metadata is missing.
  Can't initialize PV '/dev/sda' without -ff.

$ pvchange -u /dev/sda
  PV '/dev/sda' is marked as belonging to a VG but its metadata is missing.
  Can't change PV '/dev/sda' without -ff.
  Physical volume /dev/sda not changed
  0 physical volumes changed / 1 physical volume not changed

$ pvremove /dev/sda
  PV '/dev/sda' is marked as belonging to a VG but its metadata is missing.
  (If you are certain you need pvremove, then confirm by using --force twice.)

$ vgcreate vg /dev/sda
  Physical volume '/dev/sda' is marked as belonging to a VG but its metadata is missing.
  Unable to add physical volume '/dev/sda' to volume group 'vg'.