* src/twaalgos/isunamb.hh, src/twaalgos/isunamb.cc: New files.
* src/twaalgos/Makefile.am: Add them.
* src/tests/unambig.test: New file.
* src/tests/Makefile.am: Add it.
* src/bin/ltlcross.cc: Record whether each produced automaton is
ambiguous.
* src/bin/autfilt.cc: Add a --is-unambiguous option.
* NEWS: Mention it.