conf: do not check, write and delete PID file if set to '/'

conf: do not check, write and delete PID file if set to '/'

If the pid file path is specified as '/', skip handling it,
as it is not only unnecessary but complicates managing the
service. A systemd unit can manage the program without any
need for this functionality, and it makes process tracking
simpler and more robust.
The implementation matches the bindcmdaddress directive.