added STDIN Mode

May this is useful for others as well. I use this modification to ship nagios perfdata_files via a stdin redirection over ssh directly into the process_perfdata.pl script.

The example below feeds the process_perfdata.pl script on statshost with a local perfdata file:
```ssh -o 'ConnectTimeout=2' nagios@statshost.local.org '/usr/local/pnp4nagios/libexec/process_perfdata.pl -s' < perfdata_file```

Regards,
Rob