Process the X (or Nextstep) related command line options in ARGS.
This is done before the user's startup file is loaded.
Copies the options in ARGS to `x-invocation-args'. It then extracts
the X (or Nextstep) options according to the handlers defined in
`command-line-x-option-alist' (or `command-line-ns-option-alist').
For example, `x-handle-switch' handles a switch like "-fg" and its
value "black". This function returns ARGS minus the arguments that
have been processed.