Add a simple port filter to SIFTR.

SIFTR does not allow any kind of filtering, but captures every packet
processed by the TCP stack.
Often, only a specific session or service is of interest, and doing the
filtering in post-processing of the log adds to the overhead of SIFTR.

This adds a new sysctl net.inet.siftr.port_filter. When set to zero, all
packets get captured as previously. If set to any other value, only
packets where either the source or the destination ports match, are
captured in the log file.

Submitted by: Richard Scheffenegger
Reviewed by: Cheng Cui
Differential Revision: https://reviews.freebsd.org/D18897