ltl2ba.
* src/neverparse/neverclaimparse.yy: Accept multiple labels
for the same state. Honor accepting states. Forward parse
error from the parser used for guards. Accept "false" as a
single instruction for a state.
* src/neverparse/neverclaimscan.ll: Recognize "false" specifically,
and remove the ";" hack.
* src/tgba/tgbaexplicit.cc
(tgba_explicit_string::~tgba_explicit_string): Adjust not to
destroy a state twice.
* src/tgba/tgbaexplicit.hh
(tgba_explicit_string::add_state_alias): New function.
* src/tgbatest/defs.in (SPIN, LTL2BA): New variables.
* src/tgbatest/neverclaimread.test: Check error messages for
syntax errors in guards. Make sure we can read the output
of `spin -f' and `ltl2ba -f' on a few test formulae.
