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
for all platforms.
- remove now unnecessary replicated code in the various snap_md_post targets
- remove snap_md_pre; nothing uses it any more
- use ${NETBSDSRCDIR} and ${KERNSRCDIR} as appropriate
jdolecek