Populate erc--msg-prop-overrides for CTCP replies

Populate erc--msg-prop-overrides for CTCP replies

* lisp/erc/erc-backend.el (erc-server-PRIVMSG): Don't set string
intended for insertion to the undefined return value of
`erc-process-ctcp-reply' and `erc-process-ctcp-query'.  Rework control
flow slightly for clarity.
* lisp/erc/erc.el (erc-process-ctcp-reply): Bind
`erc--msg-prop-overrides' and populate with `erc--ctcp' and `erc--cmd'
"msg props" for the benefit of `erc-display-message' calls made by
the various CTCP reply handlers.  (Bug#67677)