gg: Add function to read safety status, alerts

Signed-off-by: Sean Cross <xobs@kosagi.com>
cmd-stats: Add safety alerts, safety status to output

The gas gauge reports to us various safety alerts.  These can be useful in
tracking down why FETs aren't turned on properly.

Signed-off-by: Sean Cross <xobs@kosagi.com>
cmd-stats: Print out values in either "mAh" or "mW"

Sometimes the host can switch readouts to be mW instead of mAh.  That's
not too big a deal, we just have to print out values in the new format.

Signed-off-by: Sean Cross <xobs@kosagi.com>