Don't try to unregister if we weren't registered at all

Ignore-this: e3fe24da15ce735d5b371cd11c7b87b2

darcs-hash:20100926111515-56b4d-edf5d11a15c5497721624614b0da1d3189651d96
Enhanced xcapdiff subscription termination

Ignore-this: 1f78f359d6982e5020852333fe35d48a

darcs-hash:20100926111748-56b4d-993b9e60e2cacc7fd6ac830d88aed4f34ff1c76d