Provide `pdfroff' shell script, and manpage to document it;

Provide `pdfroff' shell script, and manpage to document it;
runs multiple groff passes, to format PDF documents.

* pdfroff.sh: New shell script template;
* pdfroff.man: New man page to document it.

Integrate `pdfmark' into normal groff build system;
install macro `pdfmark' packages, build and install `pdfroff',
and PDF format documentation.

* Makefile.sub: Rewritten.
* pdfmark.tmac: Modified.
(pdfhref): New macro operators, `D' and `Z'.
(pdf*href-D, pdf*href-Z): New macros: implement them.
(pdf*href.mark.resolve, pdf*href.mark.emit, pdf*href.mark.flush):
Modified macro algorithm, to eliminate inconsistencies between
`grohtml' representations of `opminy' from differing groff versions.
(pdf*href.mark, pdf*href.mark.release, pdf*href.mark.close):
deleted (redundant macros).
(PDFHREF.LEADING): Default value changed (was 2.5p; now -1.0p).
Global comment updates.

* TODO: Updated.