fork: Document _Fork (and fork) as POSIX 2024

Also remove some information from HISTORY that is no longer needed (and
could be confusing), now that _Fork is part of a standard.

Reported by: kib
Reviewed by: imp, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47588

(cherry picked from commit 566c039d1e7555343fcf6439a10e56f5a632c0fe)