Tweak the two copies of gpt_chk_mbr() to return the index of the MBR

Tweak the two copies of gpt_chk_mbr() to return the index of the MBR
0xEE (DOSPTYP_EFI) partition, or -1 no usable such partition is found.

Adopt a consistent idiom to capture the index for future use.

Clean up the gpt_chk_mbr() logic to make it clearer what constraints
are being applied when looking for the DOSTYP_EFI partition.

No intentional functional change.