We do not require copyright assignment for contributions to CMake.
Update the top-level notice to mention "Contributors" explicitly.
Add a list of the Contributors whose names already appear in
source-level copyright notices. Find the list with the command:
$ git grep -h 'Copyright[^.]' -- *.* \
Auxiliary Modules Packaging Source Templates Tests \
| grep -v Kitware | sed 's/^#\? *//' | sort | uniq
Manually refine the list to de-duplicate entries and filter out
notices from third-party code.
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.
