[collectd] modbus plugin spamming console with debug info

[Eric Sandeen]

I hadn't noticed this before, but after running on a systemd system it
comes to the forefront:

> Message from syslogd at sandeen at Feb 24 22:39:48 ...
>  collectd:<21><00><00><00><05><01><04><02><00><02>

> Broadcast message from systemd-journald at sandeen.net (Wed 2016-02-24 22:39:48 CST):

> collectd[22528]: <22><00><00><00><05><01><04><02><00><19>

on every modbus communication.  ;)

The plugin sets modbus_set_debug(ctx, 1); - that seems a bit odd to do unconditionally.

Has anyone else run into this?  Setting to debug should almost certainly be optional,
AFAICT.

Thanks,
-Eric