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 how about just the process return code and getting the error instead of the 0/1 stuff? is there some reason to use the string code output on stdout?