The "UAHF" one used to be "more equal", mostly just because it happened
to be the first.
This makes them all equally equal.
Specifically this removes the special casing and the enum for the 201708
HF (aka BCH fork-point).
We select the right branch now purely based on the historical check-
points.
Duplicate the test_bitcoin file into the common dir,
I hope to remove the original in future.
Make the common dir a new static lib and create a new
qtestlib based unit test out of the old doublespend unit test.
TomZ