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.

doc/chrony.conf.adoc

@@ -2781,6 +2781,8 @@ e.g.:
 ----
 pidfile /run/chronyd.pid
 ----
++
+Setting this directive to _/_ disables writing and checking of the PID file.
 
 [[ptpport]]*ptpport* _port_::
 The *ptpport* directive enables *chronyd* to send and receive NTP messages