Fix regression in erc-button-add-button

Fix regression in erc-button-add-button

* lisp/erc/erc-button.el (erc-button--nick): Abide by recommended
internal naming convention and use "cusr" instead of "cuser" for
referring to an `erc-channel-user' object.
(erc-button--fallback-cmem-function,
erc-button--get-user-from-spkr-prop): Use new, preferred name
`erc-channel-members' for `erc-channel-users' table.
(erc-button-add-nickname-buttons): Use "cmem" instead of "cuser" to
refer to values of the `erc-channel-members' table, which are cons
cells, not `erc-channel-user' objects.  Use updated slot name `cusr'
when initializing `erc-button--nick' object.
(erc-button-add-button): Honor wishes of
`erc-button--modify-nick-function' advice members when they set the
`nickname-face' slot of the passed-around `erc-button--nick' object to
nil to indicate a desire to forgo adding a face while still
buttonizing the inserted nick with `erc-data', etc.  (Bug#67767)