Avoid removing a running driver's PID and socket files if the new driver process fails to initialize

[jimklimov]

Among remaining issues from #1423 to investigate:

Can we jump through some hoops to keep the original driver instance's PID file in place, if a later driver tried to start and failed to initialize? After all, this later driver process dies quickly and the original blocker remains - but now "hidden" for PIDfile lookups...

Originally posted by @jimklimov in https://github.com/networkupstools/nut/issues/1423#issuecomment-1126059223