Forgo excess nick buttonizing on JOINs and QUITs

Forgo excess nick buttonizing on JOINs and QUITs

* etc/ERC-NEWS: Mention nick-button deduping.
* lisp/erc/erc-button.el (erc-button-highlight-nick-once): New
variable to specify commands and numerics for messages that ERC should
forgo redundant buttonizing atop, assuming the presence of a
"userhost".
(erc-button--fallback-cmem-function): Change default value to
`erc-button--get-user-from-spkr-prop'.
(erc-button--get-user-from-spkr-prop): New function to derive user
object from `erc--spkr' "msg prop".  The point is to allow
`erc-speaker-from-channel-member-function' to return a display name
that differs from the speaker's actual nick as recorded by its
`erc-server-user' object.
(erc-button--get-phantom-cmem): Add `count' parameter.
(erc-button-add-nickname-buttons): Stop after first turn if the
current command appears in `erc-button-highlight-nick-once'.  Pass
iteration count to `erc-button--fallback-cmem-function'.  (Bug#67677)