portmapper/proxy: refactor proxyCommand Start()

* Spawn goroutine for command start and combined output, error
  gathering. Send these back over send-only chans.
* Blocking-read from the channels for errors, bufs and a timer signal. If we
  get an error, bomb. If we get a buf, check it with a regex for the
  magic zero proxy factor (consider replacing this mechanism, imo
  :metal:). Generate an error if we get read off the timeout chan.
@erikh is there any better way than sending back a magic value on one of the pipes from the process? seems kind of prone to breakage.