We now need only the Usage formatter to support command-line options
that print basic usage, and the supporting indented=>preformatted markup
processor to support CMake message formatting. Drop all other
documentation formatters and move the remaining code up into the top
cmDocumentationFormatter class.
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.