Support ICP and HTCP _servers_ sharing listening sockets.

Support ICP and HTCP _servers_ sharing listening sockets.

Without a shared cache, the servers will report many false misses.

ICP and HTCP _clients_ cannot be supported in SMP environment unless each
process has its own address (i.e., unique IP address and/or unique [ICP] port)
because we cannot match outgoing queries and incoming responses across
processes. If ICP/HTCP support in SMP is needed, the easiest change would
be to use random source ports for sending ICP/HTCP queries. This may also
improve security by avoiding well-known source ports for UDP queries.