Implement expires_in() for Alarm.

Useful to find out how much time is left on an alarm.
Fix first time and add to test.
Return 0 when alarm is disabled.
Set RTC just before rollover.

Time in Alarm must be kept consistent.  Any existing Alarm objects will
not work unless they are told again what the interval is.  More serious
redesign is needed and is left to another issue…
Merge pull request #270 from jediunix/alarm-expires-in

Implement expires_in() for Alarm.