Make new ctrl-c behavior "clear-commandline"

And leave the old behavior under the name "cancel-commandline".

This renames "cancel-commandline-traditional" back to
"cancel-commandline", so the old name triggers the old behavior.

Fixes #10935

doc_src/cmds/bind.rst

@@ -170,6 +170,9 @@ The following special input functions are available:
 ``capitalize-word``
     make the current word begin with a capital letter
 
+``clear-commandline``
+    empty the entire commandline
+
 ``clear-screen``
     clears the screen and redraws the prompt.