add missing '&' before 't' in url construction
make media links in rss feeds absolute

This resolves issues with broken images when the wiki feed is
hosted by third parties (e.g. FeedBurner), as mentioned in
FS#2931 comments.
add HOST and PORT to feed cache key
remove rendundant line
add security character '$' around REMOTE_USER in cache key to avoid key clashes
Merge pull request #547 from splitbrain/chris_rssfeedfixes

RSS feed fixes