DClock: test for presence of Slinky

[frankmilliron]

Driver should test for the presence of a slinky-style RAM card before attempting to unlock and read the time. The clock hardware is required to be installed onto a //c expansion RAM card to function. @qkumba suggested the following "take the value from DATA, increment, write it back, check if the new value is there". I unfortunately don't have a DClock to test, but the DClock module is occasionally triggered instead of my Cricket (Cricket is further down the jumbo chain).