Promote "normal" faces in erc-track

Promote "normal" faces in erc-track

* etc/ERC-NEWS: Add entry for new behavior involving the option
`erc-track-faces-normal-list'.
* lisp/erc/erc-button.el (erc-button-nick-default-face): New face to
serve as default for `erc-button-nickname-face'.
(erc-button-nickname-face): Change default value to
`erc-button-nick-default-face'.
* lisp/erc/erc-track.el (erc-track--massage-nick-button-faces): New
function to serve as Custom :set function for priority and "normal"
face-list options.
(erc-track-faces-normal-list): Fix Custom :type by loading
`erc-button' during validation so Customize chooses the correct UI
instead of a generic form field with "(mismatch)" printed alongside
the "STATE" button.
(erc-track-faces-priority-list, erc-track-faces-normal-list): Remove
values for "buttonized" `match' module faces that, if retained, would
need updating to feature `erc-button-nick-default-face' instead of
`erc-nick-default-face'.  However, as noted in the NEWS entry, this
ordering of button face atop match face is not possible.  Use :set
function to massage saved user values.
(erc-track-ignore-normal-contenders-p): New compatibility switch to
access pre-5.6 behavior, in which faces in
`erc-track-faces-normal-list' were only considered for promotion to
the mode line if the current face occupying that pole position wasn't
present.
(erc-track-mode, erc-track-enable, erc-track-disable): Add comments
regarding perceived futility of hooking on `erc-server-001-functions'
and likely unneeded hook removal.  Run common buffer-local setup and
teardown.
(erc-track--normal-faces): New local variable, a snapshot of
`erc-track-faces-normal-list'.
(erc-track--setup): New function to stash
`erc-track-faces-normal-list' on init.
(erc-track-select-mode-line-face): Offer alternate explanation of
certain particulars in doc string.
(erc-track--alt-normals-function): New function-valued variable to
allow other modules to intervene in deciding whether to pursue and
promote a "normal" contending face.
(erc-track--select-mode-line-face): New function similar to its public
namesake except that it considers other viable candidates among the
"normal" alternatives.
(erc-track-modified-channels): Only run face selection portion when
faces are actually found.  Use `erc-track--select-mode-line-face'
instead of `erc-track-select-mode-line-face'.
* test/lisp/erc/erc-track-tests.el
(erc-track-select-mode-line-face): New test.
(erc-track-tests--select-mode-line-face): New fixture function.
(erc-track--select-mode-line-face): New test.  (Bug#67767)