sensors plugin: Fix initialization code.

sensors plugin: Fix initialization code.

Sensors often require scaling. Also, you often have to disable them, or name
them.

The libsensors config file provides all these facilities.  Without it, the
sensor plugin is mostly useless except for some values that are often already
properly scaled by the kernel (some temperatures, some fan tachometers).

In particular, it is utterly useless for voltage monitoring outside of the 3.3V
rails.

The plugin should either evaluate the libsensors config files at startup, or it
should provide facilities to give proper names and first-order polinomial
scaling+offset.

Change-Id: I97312bee5ca33fefe846a8cdc292818d46819574
Signed-off-by: Florian Forster <octo@collectd.org>