cmake_path command: path management

Help/command/cmake_path.rst

Help/command/file.rst

Help/command/get_filename_component.rst

Help/manual/cmake-commands.7.rst

Help/release/dev/cmake_path-command.rst

Source/CMakeLists.txt

Source/Checks/cm_cxx_features.cmake

Source/Checks/cm_cxx_filesystem.cxx

Source/cmCMakePathCommand.cxx

Source/cmCMakePathCommand.h

Source/cmCommands.cxx

Tests/RunCMake/CMakeLists.txt

...path/ABSOLUTE_PATH-OUTPUT_VARIABLE-invalid-arg-result.txt

...make_path/ABSOLUTE_PATH-OUTPUT_VARIABLE-no-arg-result.txt

...nCMake/cmake_path/ABSOLUTE_PATH-unexpected-arg-result.txt

...s/RunCMake/cmake_path/ABSOLUTE_PATH-wrong-path-result.txt

Tests/RunCMake/cmake_path/ABSOLUTE_PATH.cmake

.../cmake_path/APPEND-OUTPUT_VARIABLE-invalid-arg-result.txt

...CMake/cmake_path/APPEND-OUTPUT_VARIABLE-no-arg-result.txt

Tests/RunCMake/cmake_path/APPEND-wrong-path-result.txt