* src/bin/common_output.cc, src/bin/common_output.hh: New file
with the common output code.
* src/bin/Makefile.am: Add them.
* src/bin/genltl.cc, src/bin/ltlfilt.cc: Simplify, using
argp's children parser, and calling output_formula().

* src/bin/ltlfilt.cc: Do it.

* src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh: Add a
implication() option.
* src/bin/ltlfilt.cc: Add options --implied-by, --imply, and
--equivalent-to.

* lib/error.c, lib/error.h, lib/msvc-inval.c, lib/msvc-inval.h,
lib/msvc-nothrow.c, lib/msvc-nothrow.h, m4/error.m4, m4/msvc-inval.m4,
m4/msvc-nothrow.m4: New files from gnulib
1af55d85d9762a679b4302d5995f05ccd883e956.
* lib/Makefile.am, m4/gnulib-cache.m4, m4/gnulib-comp.m4: Adjust.
* src/bin/ltlfilt.cc: Use error() and error_at_line().

* src/bin/ltlfilt.cc (main): Call set_program_name().
* lib/progname.c, lib/progname.h: New files, from gnulib
1af55d85d9762a679b4302d5995f05ccd883e956.
* lib/Makefile.am, m4/gnulib-cache.m4, m4/gnulib-comp.m4: Adjust.

* src/bin/ltlfilt.cc, src/bin/Makefile.am: New files.
* src/Makefile.am (SUBDIRS): Add bin.
* configure.ac: Add src/bin/Makefile.am.
* README: Document src/bin/.