[tcp] Do not send RST for unrecognised connections

On large networks with substantial numbers of monitoring agents,
unwanted TCP connection attempts may end up flooding iPXE's ARP cache.

Fix by silently dropping packets received for unrecognised TCP
connections.  This should not cause problems, since many firewalls
will also silently drop any such packets.

Reported-by: Jarrod Johnson <jarrod.b.johnson@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>