tcp: improve blackhole support

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.

Reviewed by: gallatin
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45304