clx-interface: Extract SET-WINDOW-SPACE-REQUIREMENT from INIT-DISPLAY.

  * This should have been done a long time ago, if not been done
this way from the start.  It's a self-contained bit of code with a
sharply-defined responsibility and boundary, and thus it should be
in its own function.

  * A straightforward extract-method refactoring, not much to see
here.  Move along, citizen.