[hdtbl] make the "hdtbl" package usable with other macro packages

  The appendix contains a fixed version of "hdtbl.tmac" and "hdtbl.misc".

  The option '-ww' can be used without being flooded with warnings.

  People can make their own diff files to see the changes.

  Was tested with the ms file "build/doc/ms.ms" (Debian
/usr/share/doc/groff-base/ms.ms.gz) by adding

.LP
.mso hdtbl.tmac
.hd*TBL .hd*TR .hd*TD "contents of the table cells" .hd*ETB

in two places in the "ms.ms" file.

(or dropping the line
.mso hdtbl.tmac
and adding the "-mhdtbl" on the command line)

using

[test-]groff -ms -t -p -e -Tpdf -M ~/tmp/hdtbl [-mhdtbl] -ww <input file> > <output>

  where -M <hdtbl directory> shows where to find the new version of the
"hdtbl.*" files.