bin: make sure that all options are in a named section

This also fixes some empty lines and unsorted options
that appeared in some tools.

* tests/sanity/bin.test: Ensure this is done.
* bin/README: Add a new paragraph about this.
* bin/autcross.cc, bin/ltlcross.cc: Move the
output options in their own section.
* bin/common_color.cc: Assume color options are
in group -15.
* bin/common_finput.cc, bin/common_finput.hh:
Add a headless variant.
* bin/genltl.cc, bin/ltlfilt.cc, bin/ltlgrind.cc,
bin/randaut.cc, bin/randltl.cc:  Do not force the
children groups, so that the options are correctly sorted.
* bin/ltlsynt.cc: Add missing groups.

bin/README

@@ -50,3 +50,10 @@ Recommendations when adding new tools or features:
      randaut -n N     means   --automata=N
   but in all cases, the intent is to specify the number of items
   to output.
+
+ - In the --help output, all options should appear in a named
+   section (like "Input options:", "Output options:"), and those
+   sections are best ordered according to one's mental view
+   of how the tool works: first, it reads the input, then
+   it processes it, then it outputs the result.  Keep --help
+   and --version at the very bottom.