Follow-up to r45174: (bug 16806) Fix regression from r44524 that caused links to...

Follow-up to r45174: (bug 16806) Fix regression from r44524 that caused links to files to not get added to mLinks in ParserOutput, which caused them to not be included in LinksUpdate::LinksUpdate(), and not added to pagelinks.

Fixes other cases broken by Parser's assumptions failing to hold after change in Title::isAlwaysKnown()'s behavior:
* Links to invalid Special: pages were being recorded, but shouldn't
* Links to valid MediaWiki: pages were no longer recorded

Instead of the NS_FILE special-case in r45174, I'm just tossing *all* isAlwaysKnown links over to ParserOutput::addLink(), and letting the latter worry about what types of titles it won't record.
Just for good measure, in case any NS_MEDIA titles make it into ParserOutput::addLink() they'll be normalized to NS_FILE.