ENH: Cleanup cmake --build interface.

This cleans up the 'cmake --build' command-line interface:
  - Rename --clean to --clean-first to better describe it.
  - Replace --extra-options with a -- separator to simplify passing of
    multiple native build tool options.
  - Document the options in the main CMake man page description of the
    --build option, and shares this with the usage message.
  - Require --build to be the first argument when present.
  - Move implementation into cmakemain where it belongs.

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