Make it possible to distinguish if a sensor value is not yet set
udisks2: Don't interpret 0K as 0C

Instead just don't set sensor value if is 0K
Fixup max and dynamic plugins for a few issues

* Ignore sensor value update when value is unset
* Immediately process a sensor when enabled to update UI faster
* Properly handle when sensor is disabled to pick new best one
Fix up some flycheck warnings