m4: Support long options.

[des]

Summary

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.

[kevans]

usr.bin/m4/main.c

64

We probably want a leading + in the short opt string to avoid the non-option permutation that rides along by default in getopt_long but not gwtopt.

[des]

Prevent permutation of non-option arguments.

[bcr]

OK for the manpages part. Thanks.

[imp]

The mandoc markup looks weird to me, but like it could work... most likely my mdoc fu is too weak.
The code itself, however, looks spot on.