Retain pid files for dead daemons in 'phd status'.

'phd status' should have a stable result when invoked multiple times.
Automatically removing PID files for dead daemons every time 'phd status' is
invoked prevents tools from noticing that a daemon has died if something
happens to invoke 'phd status' before the tool looks. This affects Puppet
noticably, since it probably runs the status command every half hour.