dsynth - Fix waitpid() bug, revamp phase poll loop

dsynth - Fix waitpid() bug, revamp phase poll loop

* Fix an issue where the wrong wait*() status is processed as
  the error code.  This could occur especially on slow machines.

* Be sure to clean out any log data on the pty while and after
  processing the wait*() on the primary pid as there might be
  some as-yet unprocessed data.

* Use non-blocking mode on the masterpty to ensure that we
  don't accidently block on it.