* src/ltlvisit/lbt.cc, src/ltlvisit/lbt.hh: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/bin/common_output.cc, src/bin/common_output.hh: Add
support for LBT output, and reporting formulae that cannot
be output in this syntax.
* src/bin/ltlfilt.cc: Pass filename and linenum to
output_formula() for better error reporting.