mdate-sh: support $SOURCE_DATE_EPOCH.

mdate-sh: support $SOURCE_DATE_EPOCH.

From https://bugs.gnu.org/77805.

* lib/mdate-sh: if $SOURCE_DATE_EPOCH is set, use it
instead of the mtime of any file. Debian was removing
@value{UPDATED}, etc., from manuals because of varying output.
Try date --date, then date -r, then perl gmtime, then
fall back to the file's mtime.
Also, warn if more than one file argument is given.
(mon_to_month): factor out conversion of Jan to January.
* t/mdate5.sh: test it.
* doc/automake.texi (Texinfo): mention it.
* NEWS: mention it.