Fwdstats serverfd (Patch)

When I did the original serverfd work for backed off connections, I assumed
serverfd=0 was invalid. This patch fixes the code so serverfd=-1 is the
default when there is no backed-off server fd.

I've also added an assert into fd_close to catch any cases where a fd is
closed while it is backed off. This condition can cause crashes later on
(Bug #1616 is an example) if not detected.

Steven

-- 
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.394 / Virus Database: 268.9.4/375 - Release Date: 25/06/2006