skytools.signal_pidfile: ignore empty pidfile, some cleanups
skytools.signal_pidfile: ignore empty pidfile, some cleanups

Empty pidfile can happen if old process failed to write it
for some reason.

tarvip/skytools,overdrive3000/skytools,overdrive3000/skytools,overdrive3000/skytools,tarvip/skytools,markokr/skytools,markokr/skytools,tarvip/skytools,markokr/skytools