Add check for net.inet.tcp.blackhole and warn users, when it is set, as well
as disable socket.test in this case. Some of the TCL's self-tests attempt to
open a socket to (or request http-data from) a "dead" port on the localhost.
These tests expect a quick failure, but get a hang, when the blackhole is on.
Tests in http.test use a timeout, but some of those in socket.test do not...
Pointed out by: confusion on ports@