prevent RST spoofing

This fixes CVE-2004-0230 (http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2004-0230)
by sending an ACK instead of closing the connection, when an RST packet is received but does not match the expected sequence number.
If it only matches into the TCP receive window, we send an ACK.
A legimitate RST packet with matching sequence number should follow, otherwise it was probably a spoofing attack by someone guessing a sequencenumber inside the window.

Closes #14