Use gnulib manywarnings module and fix warnings it produces.
Remove is_cwfile.

This function was used for two purposes: first, to check we don't
fork-bomb, but that would require writing a bogus path directive. Not
very likely. Secondly, it was used as a check before trying to parse a
definition file, but that's silly: a valid definition file is one we
can parse. There's no reason in principle why you shouldn't want
non-executable definition files that don't start with a hash-bang
line.
.gitignore: add TAGS
Fix command-line help.

addhelp_display should not call convert_string, because the coloring
system may not yet have been initialised. If we want colour output
from cw, use it to wrap itself.

Don't print the error about cw being a non-interactive program unless
we tried to run it with no command-line arguments at all, so that "cw
--help" works despite having no definition argument.

Remove -V and remaining documentation about supermaps.
Remove some remaining support for cat ('@') files.