[Twisted-Python] cReactor fixes

[Brian Warner]

> I'll fix the test suite to only run these tests if the appropriate
> interface is implemented. There's no reason cReactor has to support UDP
> or multicast, though it'd be nice if it implemented IFDSelector or
> whatever it's called so that it could use the python UDP, SSL, process
> and multicast code.

You might want to provide an easy way to turn those tests back on (bypassing
the "is-it-implemented" test). Unit tests are a good way to point out
functionality that hasn't been implemented yet. If the test suite appears to
pass cleanly, someone might be fooled into thinking that cReactor works just
as well as the others.

Of course, there might be other/better ways to signal that this reactor
behaves differently than all the others, and that this is a design decision
and not just as-yet-unimplemented functionality. (__implements__ comes to
mind, but I don't know how visible that would be to a user who's left
wondering why their program fails when run under cReactor).

Is there anyone here who is using cReactor? If so, what is important to you
about it? Is UDP a burning desire?

thanks,
 -Brian