A bunch of improvements:

* don't hold spc mutex while sending data
* use send() for the banner to avoid SIGPIPE in case a client
  connects and immediately goes away
* fix error path locking
* use kevent() instead of pollts() in the client.  Apparently that
  is the only sensible way for a library to support both multithreading
  and signal-reentrancy in a race-free manner.
  (can I catch all signals with one kevent instead of installing
  NSIG different ones??)
* mark client comm descriptor non-blocking so that clients have
  better signal-interruptibility (we now sleep in signal-accepting
  kevent() instead of signal-masked recvfrom())

(pooka)