Add missing ta=4.2BSD to the `a' partition of `floppy' entry.
Okay'ed by jdolecek.

use single minimal default disktab file for most archs, replacing
MD disktab where possible (everything but vax)
the MD disktab was often either empty, or contained entries even for disks
which support geometry autodetection (SCSI), and/or non-interesting/long
obsolete entries

the old records are still available in example disktab in
/usr/share/examples/disktab/disktab in case anyone would need them