Honour /usr/mdec/mbr or -f partition 0 information even outside

Honour /usr/mdec/mbr or -f partition 0 information even outside
of _powerpc__ and __mips__, preventing the possible creation of
MBRs with overlapping partitions 0 and 3.

Don't set bootprt DOSACTIVE flag if no partition 0 information
was provided via /usr/mdec/mbr or -f. Lets octeon fdisk(8) set
DOSACTIVE on the OpenBSD partition in the default MBR like
every other fdisk(8) does.