1046132: Makes rhsm-icon slightly less annoying.

This adds a static StatusType last_status used to prevent rhsm-icon from displaying the same notification every time the status is updated via dbus. Changes have been made to ensure rhsm-icon will pop up on the given check_period as long as the new status is not RHSM_VALID. 

While this does not prevent rhsm-icon from being displayed while the gui is displayed it does reduce the number of times it will pop up down to two cases:
1) when the check_period timeout occurs.
2) when the entitlement status changes
difftime returns a double. why use long for time_since_last_check?

Why time(0) instead of time(NULL) ?