Fix the MSDOS build broken by last revisions.

 msdos/sed1v2.inp: Use $(..) instead of ${..} in all edit commands.
 Needed because of changes in 2010-10-10T14:43:05Z!dann@ics.uci.edu.
 msdos/sed6.inp (mkinfodir): Edit to avoid Unix shell-isms.  Needed
 because of changes in 2010-10-09T18:31:12Z!rgm@gnu.org.
 src/unexcoff.c (make_hdr): Fix prototype according to changes in
 2010-10-03T13:59:56Z!dann@ics.uci.edu.