m4: Support long options.

The option names are identical to those of the equivalent options in GNU m4, except of course for --gnu which does not exist in GNU m4.

While here, make the argument to -d / --debug optional, with the same default behavior as in GNU m4, and document it properly.

Sponsored by: Klara, Inc.
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D40694