Shouldn't we check whether these extant pidfiles are just stale? If my celery dies uncleanly, then I issue a <code>start</code>, I will be unhappy if it reports "celeryd is already running, here are the (stale) pidfiles to prove it."

This can be worked around by running a <code>stop</code> first, which isn't so bad, but I think we can tidy this up.