Fixes to FavIcon handling.  Image cache now behaves more like cache.

A feed update will re-fetch the feed image if the current image cache file does not exist on disk.

Quelled 'undefined offset' notices arising from using list()=explode() to parse strings.
Check more variables for null values before trying to use them.
Simplified link-element processing.
FavIcon absolute urls are now constructed correctly.

Removed redundant fetch when storing favicon data into cache.