This converts the CMake license to a pure 3-clause OSI-approved BSD
License. We drop the previous license clause requiring modified
versions to be plainly marked. We also update the CMake copyright to
cover the full development time range.
The "Keep only FinalPass commands in memory" commit caused instances of
this command to be deleted after the InitialPass. Even though the
variable_watch command does not have a final pass, it does need to stay
alive because it owns the callback information.
