Have the GPT probe return a lower priority when the MBR is not a PMBR

Have the GPT probe return a lower priority when the MBR is not a PMBR
The purpose of the PMBR is to have the disk appear in use to GPT
unaware utilities (like fdisk).  However, if the PMBR has been changed
by a GPT unaware utlity then we must assume that this was deliberate
(as it involved removal of the special slice) and we should not treat
the unmodified GPT-specific sectors as being valid.  By lowering the
probe priority in that case, the MBR scheme will take precedence and
the kernel will end up using the MBR and not the GPT. We will still
use the GPT if the kernel does not support the MBR scheme.