Add a new command for mode-specific commands

Add a new command for mode-specific commands

* doc/lispref/commands.texi (Interactive Call): Document it.
* lisp/simple.el (command-completion-using-modes-p): Refactored
out into its own function for reuse...
(command-completion-default-include-p): ... from here.
(execute-extended-command-for-buffer): New command and keystroke
(`M-S-x').