doc/Gbk-1pg-sfx.mn is already using one half of a sound technique: set
the page length to a very large value, guaranteed to overshoot the
vertical space required by the document's text. The other half is to,
at the end of the document, set the page length to the current vertical
position, so that it ends immediately.
https://www.gnu.org/software/groff/manual/groff.html.node/Manipulating-Spacing.html
A typo in Gbk-1pg-sfx.mn resulted in " /" as a "last word" on the
end-of-data marker line of the intermediate 12345 line single page.
That was effectively invisible because it immediately got deleted
during the post-processing which strips off that marker through to
the end (producing a 5266 line single page at present).
Update sys/unix/Makefile.doc to be able to build Guidebook.dat, an
unpaginated copy of the plain text Guidebook. It's generated from
the 'roff source rather than by attempting to post-process the
regular text Guidebook.
No doubt there are easier ways to do this.
