This implements the first point of issue #529.
* spot/tl/apcollect.cc, spot/tl/apcollect.hh (collect_litterals): New
function.
* bin/ltlsynt.cc: Implement the --polarity option, use
collect_litterals() to simplify the specification, finally patch the
game, Mealy, or Aiger output.
* spot/twaalgos/aiger.cc, spot/twaalgos/aiger.hh: Take a
relabeling_map has argument to specify extra APs.
* tests/core/ltlsynt.test, tests/core/ltlsynt2.test: Adjust test
cases.