No semantic changes

{memory,lockstep}_stream_pair are more useful

Now that we have a clear delineation between the high-level and
low-level interfaces, this should go into the high-level interface.

No more port=None or port="" or port=bytestring -- they're not useful
and they can have weird/non-portable behavior.

Also don't accept service names (port="http") -- it would work, but
they aren't necessarily portable, I can't find evidence that anyone
uses them, and they violate TOOWTDI. I'd rather not support things I
wouldn't recommend using.

Also don't allow host=None in open_tcp_stream -- this is redundant
with the loopback address. I guess this *could* be useful in some
weird case, but we can always add it back later if someone wants it.