Update ipmi_sensor_ to handle ipmitool Unable to read sensor errors

[smokris]

Occasionally (once every few days) ipmitool will output a reading like this:

Sensor Reading        :  Unable to read sensor: Device Not Present

Currently, Munin's ipmi_sensor_ produces the following output in that situation:

fan1.value Unable

…which causes Munin to log:

Failed to interpret expected numeric value of field 'fan1' (host 'palo.kosada.com'): 'Unable'
Argument "Unable" isn't numeric in sprintf

…and issue alerts as though the reading had a value of 0.

This PR modifies ipmi_sensor_ to omit the invalid value, which should avoid the unnecessary alerts.

[kenyon]

Returning a value of U is the correct thing to do in this situation, which means an unknown/undefined value at that time. This is documented in https://guide.munin-monitoring.org/en/latest/develop/plugins/plugin-concise.html#fetch.

[smokris]

Ah, thanks. I just amended it to output fan1.value U when the sensor reading is unavailable.