On FreeBSD there is a setsockopt option SO_USER_COOKIE which allows
setting a 32 bit value on each socket. This can be used by applications
and DTrace as a rendezvous point so that an applicaton's data can
more easily be captured at run time. Expose the user cookie via
DTrace by updating the translator in tcp.d and add a quick test
program, a TCP server, that sets the cookie on each connection
accepted.

Reviewed by: hiren
MFC after: 1 week
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D7152