9bootfat: only check for fat at block 0 on floppy drives (thanks aap)

smart boot manager has a "FAT" signature in its mbr causing
9bootfat to "detect" it as a fat filesystem and then fails
to find plan9.ini.

there shouldnt be a fat filesystem on harddrives at block 0, only
on floppy drives. but some bioses use floppy drive numbers
for usb harddrives so still check for a partition table.

thanks aap for debugging this.

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