help to solve the problem with mkextrafs, but it turns out that FBSD4 and 5
have different ideas about the geometry of the same disk.  So a FBSD4 MFS
(running growdisk) booting a FBSD5 system (running mkextrafs) will exhibit
a problem as will the reverse.

Bah!  Time to punt and change mkextrafs to not use fdisk just to change
the partition type.  This cleans up an ugly hack in the Linux version of
mkextrafs as well.