[-] MO gsitemap: fix incorrect lastmod (closing #PNM-1787)

Was using date('c') or system time for all entries. Now using date_upd from DB when available:

product
category
manufacturer
supplier
* no date_upd available in DB for CMS pages

If no date_upd available then module already defaults to "dunno" and skips lastmod tag. This preserves priority for pages that really have been changed more recently as opposed to giving current time to media we don't even know about.