lophttpd caching trickery

I added 404-caching to lophttpd, in the hope that
it will result in overall speedup if a lot of clients
try to fetch non-existing files (favicon.ico etc.)

The key for the cache is the first line that is sent,
so once we see this request again, we dont need to parse the
entire header, decode the pathname and stat() it before we send
the error reply. Good, eh?