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
check_period is seconds (and well below the maximum integer value on 32 or 64 bit systems)
time_since_last_check (from difftime) represents the difference in seconds between the two times. (double-> long doesn't make that much sense, but definitely shouldn't break anything)

There shouldn't be any fractions anyhow because I believe we're using a 64-bit unsigned int for time, however that may not be true for every arch we support.  Either way the cast should be handled without any problems.

last_status initializes to the 0 value of the enum, which represents "fully valid".  If we get the same status again, we don't display anything (we hide the icon).  If it is one that we show, the enums don't match and the icon will be displayed.