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

(cherry picked from commit 47b32f8f93b46ce2c7fe3973c331a40096b529da)

m4: Add -G option which reverses -g.

This matches GNU m4's -G / --traditional option, and although BSD m4 in non-GNU mode is not exactly identical to GNU mode in traditional mode, it's close enough.

Sponsored by: Klara, Inc.
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D40697

(cherry picked from commit 6c9ed64c94f192eda8c8b5e9e2b92ff93da36de2)

m4: Miscellaneous man page fixes.

Sponsored by: Klara, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40701

(cherry picked from commit db4c5b8a05bbd54a00dad145a91902d71c1f92a1)

Fix spelling of "behavior" to be consistent with the rest of the file

(cherry picked from commit 764464af49688e74fd6d803df0404ca4726dd460)