1. Tail --follow file in case Poller did not finish yet.
2. Do not tail --follow if Poller crashed.
3. Expose Poller run time and number of hosts and RRDs processed.

'==' can not compare file handles. We can use fileno() function to
compare numerically file numbers. Code without this returns
"connection successful" when it timeouts and $fd is "undefined".

Added option -r <pidfile> for resmon.
If none provided it will run without creating pidfile.
If provided, it will try write it's pid into specified file.
Resmon-systemd.service should have this file as an argument for
resmon as a PIDFile option and in ExecStart option.