To ease debugging and testing, ltlsynt can output the synthesized
strategy as an automaton, not just an aiger circuit.
Also, its exit code has been changed to something meaningful.
* bin/ltlsynt.cc: Various improvements: options, exit code, code style
* spot/twaalgos/aiger.hh, spot/twaalgos/aiger.cc,
spot/twaalgos/Makefile.am: Move the aiger printer to separate files
* tests/core/ltlsynt.test: Clean up and update test file
* tests/Makefile.am: Add the test file to the test suite
* NEWS: document the new aiger printer
* doc/org/concepts.org: document the named property "synthesis-outputs",
used by print_aiger
