Fixes#400.
* spot/tl/randomltl.cc, spot/tl/randomltl.hh: Adjust to accept
a set of formula to replace the atomic propositions.
* bin/ltlmix.cc: New file.
* bin/Makefile.am: Add it.
* bin/man/ltlmix.x: New file.
* bin/man/Makefile.am: Add it.
* doc/org/ltlmix.org: New file.
* doc/Makefile.am: Add it.
* bin/man/genltl.x, bin/man/randltl.x, bin/man/spot.x, bin/spot.cc,
doc/org/arch.tex, doc/org/concepts.org, doc/org/tools.org, NEWS: Mention
ltlmix.
* tests/core/ltlmix.test: New file.
* tests/Makefile.am: Add it.
The \f(CW macro to switch to "constant-width" does not seem to
honored when converting to html, and I've found some patch to
groff removing its use from their own man page.
https://lists.gnu.org/archive/html/groff-commit/2020-07/msg00015.html
Lets use \fC instead, as it seems to produce some <tt> in HTML.
Two manpages had URLs pointing to spot.lrde.epita.fr instead of
spot.lre.epita.fr.
Finally, spot-x.x had an incorrectly closed .EX block, that completly
broke the HTML conversion.
* bin/man/autcross.x, bin/man/ltl2tgba.x, bin/man/ltlcross.x,
bin/man/spot-x.x, bin/man/spot.x: Fix the aforementioned issues.
