Feature: Add option to indicate syslog originating host

[TScalzott]

The monitor can be used as a syslog forwarder by allowing remote connections with "accept_remote_connections:true"

However, all forwarded entries will log with $serverHost equal to where the agent is running. An option to override with the name/ip of the syslog entry's sender would allow proper searching for and on those devices.

Alternatively, populating a new variable such as $sendingHost would be acceptable.

There may be a need to take reverse name resolution into account for performance reasons.