This improves gf_guarantee_to_ba() on formulas GF(φ) where the
automaton for F(φ) as several leading transiant SCCs. E.g.,
GF(a <-> XXXa) where we know get results that are as good as
those of delag without loosing on the cases where delag's technique
would actually produce two big automata.
* spot/twaalgos/gfguarantee.cc: Implement this.
* spot/twaalgos/gfguarantee.hh, NEWS: Document it.
* tests/core/ltl2tgba2.test, tests/core/ltl3ba.test: Add test cases.
* bin/common_trans.cc, bin/common_trans.hh: Add the --relabel option.
* bin/ltlcross.cc, bin/ltldo.cc: Implement it.
* doc/org/ltldo.org, NEWS: Document it.
* tests/core/ltl3ba.test: Test it.
