branches: 1.1.2; 1.1.6; 1.1.28;
Rework /usr/share/doc.

Update the <bsd.doc.mk> infrastructure, and update the docs to match
the new infrastructure.

- Build and install text, ps, pdf, and/or html, not roff sources.

- Don't wire the chapter numbers into the build system, or use them in
the installed pathnames. This didn't matter much when the docs were a
museum, but now that we're theoretically going to start maintaining
them again, we're going to add and remove documents periodically and
having the chapter numbers baked in creates a lot of thrashing for no
purpose.

- Specify the document name explicitly, rather than implicitly in a
path. Use this name (instead of other random strings) as the name
of the installed files.

- Specify the document section, which is the subdirectory of
/usr/share/doc to install into.

- Allow multiple subdocuments. (That is, multiple documents in one
output directory.)

- Enumerate the .png files groff emits along with html so they can be
installed.

- Remove assorted hand-rolled rules for running roff and roff widgetry
and add enough variable settings to make these unnecessary. This
includes support for
- explicit use of soelim
- refer
- tbl
- pic
- eqn

- Forcibly apply at least minimal amounts of sanity to certain
autogenerated roff files.

- Don't exclude USD.doc, SMM.doc, and PSD.doc directories from the
build, as they now actually do stuff.

Note: currently we can't generate pdf. This turns out to be a
nontrivial problem with no immediate solution forthcoming. So for now,
as a workaround, install compressed .ps as the printable form.