Added support for "oneshot" modbus requests

[fre-ros]

I added functionality for a "oneshot" modbus command that always sends one modbus requests instead of polling with an interval.
To trigger a oneshot command one should pass the option modbus_oneshot_timeout_ms.

If a timeout happens the module now returns a response with tag modbus_error and string value oneshot_timeout. I feel that maybe there is a better way to name these fields, what do you think?

• Added new functionality for oneshot command inp_modbus.c
• Fixed some incorrect printf format strings in modbus_poll_callback
• Updated man page

[fre-ros]

After some more testing something seems amiss.

If I disconnect my modbus server from the network then rrr will only give me the "queued" modbus_oneshot_status and not "timeout" for around 30 seconds when I do new requests. After that the "timeout" status appears.

So for some reason p_modbus.c doesn't send the timeout status for the first 30 seconds when the modbus server is unresponsive.

[fre-ros]

After some more testing something seems amiss.

If I disconnect my modbus server from the network then rrr will only give me the "queued" modbus_oneshot_status and not "timeout" for around 30 seconds when I do new requests. After that the "timeout" status appears.

So for some reason p_modbus.c doesn't send the timeout status for the first 30 seconds when the modbus server is unresponsive.

After our email conversation this is not an issue, so I think this is ready for merge.