Add add_cmd function overloads

This adds two add_cmd overloads: one whose callback takes a const char *,
and one that doesn't accept a function at all.  The no-function overload
was introduced to avoid ambiguity when NULL was passed as the function.

Long term the goal is for all commands to take const arguments, and
for the non-const variants to be removed entirely.

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* cli/cli-decode.c (add_cmd, set_cmd_cfunc): New function
	overloads.
	(do_add_cmd): Rename from add_cmd.  Don't call set_cmd_cfunc.
	(do_const_cfunc): New function.
	(cmd_cfunc_eq): New overload.
	(cli_user_command_p): Check do_const_cfunc.
	* cli/cli-decode.h (struct cmd_list_element) <function>: New field
	const_cfunc.
	* command.h (add_cmd): Add const overload and no-function
	overload.
	(set_cmd_cfunc): Add const overload.
	(cmd_const_cfunc_ftype): Declare.
	(cmd_cfunc_eq): Add const overload.
	* breakpoint.c, cli-cmds.c, cli-dump.c, guile/scm-cmd.c,
	python/py-cmd.c, target.c, tracepoint.c: Use no-function add_cmd
	overload.

gdb/cli/cli-decode.h

@@ -114,6 +114,8 @@ struct cmd_list_element
       {
 	/* If type is not_set_cmd, call it like this: */
 	cmd_cfunc_ftype *cfunc;
+	/* ... or like this.  */
+	cmd_const_cfunc_ftype *const_cfunc;
 	/* If type is set_cmd or show_cmd, first set the variables,
 	   and then call this: */
 	cmd_sfunc_ftype *sfunc;