Create a fake geometry (16 heads, 64 sectors) when dealing with
a plain file and a geometry is not explicitly supplied through
command line or disktab entry.

This way you can a FAT image on a file as simply as this:

newfs_msdos ./some/file

(right now you need a much longer command

newfs_msdos -h 32 -u 64 -S 512 -s $total_blocks -o 0 ./some/file

Will be merged after 7.1 and 6.4 are released.
See also the related PR which suggests a similar change.

PR: bin/121182
MFC after: 4 weeks