tcp: improve blackhole support

[tuexen]

Summary

There are two improvements to the TCP blackhole support:

1. If net.inet.tcp.blackhole is set to 2, also sent no RST whenever a segment is received on an existing closed socket or if there is a port mismatch when using UDP encapsulation.
2. If net.inet.tcp.blackhole is set to 3, no RST segment is sent in response to incoming segments on closed sockets or in response to unexpected segments on listening sockets.

Thanks to gallatin@ for suggesting such an improvement.