Add the connect-request and connect-established probes.

One caveat: the connect-established probe doesn't fire when a connection
goes through SYN-SENT -> SYN-RECEIVED -> ESTABLISHED via a simultaneous
open. The Solaris docs say that the probe should indeed fire in this
case once the connection is in the ESTABLISHED state; however, I don't
see any way to determine from the tcpcb whether the connection was
opened actively or passively. illumos has a flag tcp_active_open flag
in their tcbcb equivalent, so they can just check it when going from
SYN-RECEIVED to ESTABLISHED.