Sequester some special-variable declarations in ERC

Sequester some special-variable declarations in ERC

* lisp/erc/erc-backend.el (erc-log-p): Remove declaration entirely.
(erc-server-reconnect): Move declaration for `erc-reuse-buffers' here.
(erc-process-sentinel-1): Move `erc-kill-server-buffer-on-quit'
declaration here.
(erc--conceal-prompt): Move `erc-prompt-hidden' declaration here.
(erc-server-INVITE): Move `erc-invitation' declaration here.
(erc-server-PART): Move `erc-kill-buffer-on-part' declaration here.
(erc-server-PRIVMSG): Move declarations for `erc-minibuffer-ignored',
`erc-receive-query-display', `erc-receive-query-display-defer',
`erc--cmem-from-nick-function', `erc-format-nick-function', and
`erc-format-query-as-channel-p' here.
(erc-server-401): Move `erc-whowas-on-nosuchnick' declaration here.
(erc-server-475): Move `erc--called-as-input-p' and
`erc-prompt-for-channel-key' declarations here.
* lisp/erc/erc-common.el (erc-log-p): Remove declaration for
`erc-log-p' and replace with actual definition.
(erc-log-aux): Move `erc-dbuf' declaration from top level into
function body.
* lisp/erc/erc-fill.el (erc-fill--wrap-continued-message-p): Note in
the doc string that this function produces side effects.
* lisp/erc/erc-networks.el (erc-determine-network,
erc-networks--copy-server-buffer-functions): Move some variable
declarations to function body.
(erc-settings, erc-get): Deprecate for now and explain why in doc
strings.  We could deprecate them unconditionally, but they never
provided usable code, and their names are short and valuable.
* lisp/erc/erc.el (tabbar--local-hlf, motif-version-string,
gtk-version-string): Prefer moving these single-serving declarations
to function bodies, if only to make closures from this library less
cluttered when debugging.  This should also help avoid stray
declarations.
(erc-hooks, erc-timer-hook): Revise doc strings.
(erc-log-p): Move definition to erc-common.el.
(erc-cmd-SV): Mimic `emacs-version' and stick with `featurep', here in
combination with special-variable declarations, instead of `boundp' or
similar.
(erc-header-line-uses-tabbar-p): Explain that this has nothing to do
with `tab-bar'.
(erc-update-mode-line-buffer): Only assign when bound.  This has been
verified to work with version 2.0 on EmacsWiki.
* test/lisp/erc/erc-scenarios-base-reconnect.el: Timeouts.
* test/lisp/erc/erc-scenarios-base-renick.el: Timeouts.
* test/lisp/erc/resources/join/network-id/foonet-again.eld: Timeouts.