Send RST packet when closing an ICAP connection after a transaction error.

Send RST packet when closing an ICAP connection after a transaction error.

This helps identify abnormal transaction termination at the ICAP server. The
code is from a production-running branch and was proven useful in some
environments (especially if the ICAP service needs help correctly dealing with
prematurely terminated transfers), but it is not yet clear whether the
advantages of doing this on a regular basis outweigh the overheads. If not,
we will need a squid.conf parameter to control connection closing behavior.