libblkid: detect alone PMBR

wipefs(8) has to also erase PMBR, not GPR header only, otherwise the
device could be still interpreted as a device with a partition table.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1054186
Signed-off-by: Karel Zak <kzak@redhat.com>
wipefs: call BLKRRPART when erase partition table

It's better to be smart than make things inconsistent (without
BLKRRPART kernel still uses the erased PT and udev-db still contains
obsolete information).

Signed-off-by: Karel Zak <kzak@redhat.com>