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/randaut.cc

@@ -128,7 +128,7 @@ static const struct argp_child children[] =
   {
     { &aoutput_argp, 0, nullptr, 3 },
     { &aoutput_o_format_argp, 0, nullptr, 4 },
-    { &misc_argp, 0, nullptr, -1 },
+    { &misc_argp, 0, nullptr, 0 },
     { nullptr, 0, nullptr, 0 }
   };