Fixed a race condition in service-signal testcase

The problem was that killpg() was happening before the child
had called setpgrp(), and so the process group didn't exist.

Fixed it by enhancing the DaemonProxy perl test module to also
set up a pipe for stderr, and then connect the script to stderr
and print 'ready'.  I then wait for both 'service up' from
daemonproxy and 'ready' from child before calling service.signal