The vim, emacs, and bash support files are not internal CMake resources
and so do not belong under `CMAKE_DATA_DIR`. Move them over to proper
places under the `CMAKE_XDGDATA_DIR` as we do already for cmake-gui
desktop files and `cmake.m4`.
Fixes: #20522
In case of long '<command> <args...>' the description text is wrapped
and indented on the next line.
Avoid taking these lines into account by explicitly requiring the third
character to be a non-space.
With the current version of homebrew one gets:
bash: _init_completion: command not found
Avoid this by initializing manually when the function is not defined.
