Improve ltlsynt interface

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

NEWS

@@ -44,8 +44,8 @@ New in spot 2.4.2.dev (not yet released)
     also be used to detect unreliable measurements. See
       https://spot.lrde.epita.fr/oaut.html#timing
 
-  - ltlsynt is a new tool for synthesizing AIGER circuits from LTL/PSL
-    formulas.
+  - ltlsynt is a new tool for synthesizing a controller from LTL/PSL
+    specifications.
 
   - ltldo learned to limit the number of automata it outputs using -n.
 
@@ -190,6 +190,15 @@ New in spot 2.4.2.dev (not yet released)
   - The new spot::formula::is_leaf() method can be used to detect
     formulas without children (atomic propositions, or constants).
 
+  - The new function spot::print_aiger() encodes an automaton as an
+    AIGER circuit and prints it. This is only possible for automata
+    whose acceptance condition is trivial. It relies on a new named
+    property "synthesis outputs" that describes which atomic
+    propositions are to be encoded as outputs of the circuits.
+    This function is used by ltlsynt to output the synthesized
+    controllers in the format required by the synthesis tools
+    competition SYNTCOMP.
+
   Deprecation notices:
 
   (These functions still work but compilers emit warnings.)