Add format-catalog entry for unknown chan mode in ERC

Add format-catalog entry for unknown chan mode in ERC

* lisp/erc/erc.el (erc--process-channel-modes): Use format spec
catalog entry, and convert char to string.
(erc-message-english-channel-mode-unknown): New variable.
;; * test/lisp/erc/erc-scenarios-base-renick.el
;; (erc-scenarios-base-renick-queries-solo): Adjust timeout.
;; * test/lisp/erc/erc-scenarios-ignore.el (erc-scenarios-ignore/basic):
;; Adjust timeouts.
;; * test/lisp/erc/erc-scenarios-misc.el (erc-scenarios-base-flood):
;; Adjust timeouts.
;; * test/lisp/erc/resources/base/assoc/bouncer-history/barnet.eld:
;; Adjust timeouts.
;; * test/lisp/erc/resources/base/reuse-buffers/channel/barnet.eld:
;; Adjust timeouts.
;; * test/lisp/erc/resources/erc-d/erc-d-tests.el (erc-d-run-basic):
;; Adjust timeouts.
;; * test/lisp/erc/resources/sasl/plain.eld: Adjust timeouts.
;; * test/lisp/erc/resources/sasl/external.eld: Adjust timeout.
;; * test/lisp/erc/resources/base/auth-source/foonet.eld: Adjust
;; timeout.

(cherry picked from commit caddc4e727a46e6b6ec7146c9e1cdc7b954f5c16)