old_text conversion on load, for future UTF-8 conversion of en.wikipedia.org and...

old_text conversion on load, for future UTF-8 conversion of en.wikipedia.org and remaining holdouts.
* When not $wgUseLatin1, saved revisions are now marked with 'utf-8' in old_flags
* When $wgLegacyEncoding is set, an old row without 'utf-8' in old_flags is converted from $wgLegacyCharset to $wgInputEncoding at load time (after gzip decompression if applicable). Thus the old_text fields will not need to be modified when the wiki is converted; this will mainly be useful after the SCHEMA_WORK changes are made so that the other fields in the old table are discarded (they will need to be separately converted anyway)