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.
This creates cmCTestGIT to drive CTest Update handling on git-based work
trees. Currently we always update to the head of the remote tracking
branch (git pull), so the nightly start time is ignored for Nightly
builds. A later change will address this. See issue #6994.