much time when the formula is large, and it is useless when the
purpose is model-checking with Spin.

* src/tgbaalgos/neverclaim.hh (never_claim_reachable): Add the
comments option.
* src/tgbaalgos/neverclaim.cc (never_claim_bfs,
never_claim_reachable): Honor the comment option.
* src/tgba/tests/ltl2tgba.cc (-N): Do not comment states.
(-NN) New option to output a commented never claim.