Avoid "control-control-KEY" (bug#55738)

Avoid "control-control-KEY" (bug#55738)

Constructs such as ?\C-^@ or ?\C-\C-m literally apply a Control
modifier twice which doesn't make sense at all.  What is really meant
is a C0 base character with the Control modifier bit set.
This change is only stylistic in nature.

* lisp/edmacro.el (edmacro-format-keys):
* lisp/keymap.el (key-parse):
* lisp/subr.el (event-modifiers, event-basic-type):
* test/lisp/subr-tests.el (subr-test-kbd):
Use \0 and \r instead of ^@ and \C-m to represent NUL and RET
when combined with other modifiers.