Add a mechanism for querying before executing a command

Add a mechanism for querying before executing a command

* doc/emacs/custom.texi (Disabling): Document it.
* doc/lispref/commands.texi (Disabling Commands): Document the
low-level stuff.

* lisp/simple.el (command-execute): Respect the `(query ...)'
value for `disabled'.
(command-execute--query): New function.
(command-query): New function.