Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r226422, r227241, r228063, r228697, r228698, r228701, r234310, r234850,
r250226, r250926.

This brings m4 up to date with the version in head and fixes the eval
command when called with a 2nd and 3rd argument. The only differences
remaining are related to the updated flex and byacc in head.

PR: bin/166994

Reimplement support for the ** (exponent) gnu extension, make it available thought the -g (mimic gnu) option

Reviewed by: cognet
Approved by: cognet
Discussed with: espie@OpenBSD.org (upstream)