Provide a way to output automata with state-based acceptance.  Also
print the guards using to_lbt_string() for consistency: as a
consequence, atomic proposition that do not match p[0-9]+ are now
double-quoted.

* src/tgbaalgos/lbtt.hh (lbtt_reachable): Add a sba option.
* src/tgbaalgos/lbtt.cc: Implement it, and use to_lbt_string().
* src/ltlvisit/lbt.cc (is_pnum): Reject 'p' without number.
* src/bin/ltl2tgba.cc: Activate the sba option of --ba was given.
Add an option --lbtt=t to get the old behavior.
* src/bin/man/ltl2tgba.x: Document the LBTT format we use with
some links and examples.
* src/tgbatest/lbttparse.test: More tests.
* src/tgbatest/ltlcross2.test: Add a check with --lbtt --ba.
* NEWS: Update.