Fix the MSDOS build broken by last revisions.

 msdos/sed1v2.inp: Use $(..) instead of ${..} in all edit commands.
 Needed because of changes in revno 101897.
 msdos/sed6.inp (mkinfodir): Edit to avoid Unix shell-isms.  Needed
 because of changes in revno 101876.
 src/unexcoff.c (make_hdr): Fix prototype according to changes in
 revno 101757.