Convert the lwp list into a red-black tree. This greatly reduces the

Convert the lwp list into a red-black tree.  This greatly reduces the
overhead of looking up LWPs for numerous operations including select and
removes the hokey fork code that tried to avoid doing a list traversal.

One inefficiency remains which cannot be easily fixed, and may not matter
much anyway, and that is delivering a generic signal the process may have
to iterate through many LWPs before finding one that can handle the signal.