MFC r306740, r307150: groff: use changelog date in man pages
r306740:
groff: use changelog date rather than file modification date in man pages
The source checkout date is not particularly relevant, and this makes
groff man pages build reproducibly.
r307150:
Avoid using 'head' in generating groff doc date
It may not be available in certain cross build cases.
Note that this is a slight change in functionality, in that now only the
first line of the source ChangeLog file is processed. This is acceptable
as groff will be retired and we won't encounter a possibly-different
ChangeLog format.
Sponsored by: The FreeBSD Foundation