Set the alarm on five minute boundaries.

There's normally no reason to set an alarm more accurately than five
minutes, so do that.

When switching between normal and decimal modes, select the closest
five minute boundary in the new mode.  In trips back and forth between
modes, the time can sometimes change once on the first round trip, but
will remain stable after that.

Signed-off-by: Russell Steicke <russells@adelie.cx>