* lisp/mail/: Use lexical-binding

* lisp/mail/: Use lexical-binding

Remove some redundant `:group` args as well.

* lisp/mail/supercite.el: Use lexical-binding.
(completer-disable): Declare var.
(sc-set-variable): Don't rely on dynbind to access `help` variable.

* lisp/mail/mail-extr.el: Use lexical-binding.
(mail-extract-address-components): Avoid use of dynamic scoping to
refer to local vars.

* lisp/mail/mailabbrev.el: Use lexical-binding.
(mail-abbrev-make-syntax-table): Rename `_` variable to `syntax-_`.

* lisp/mail/mailheader.el: Use lexical-binding.
(headers): Don't declare as dynbound globally.
(mail-header-set, mail-header-merge): Declare `headers` as dynbound
locally, instead.  Mark those functions as obsolete.
(mail-header-format): Use `alist-get` instead of `mail-header`.

* lisp/mail/binhex.el (binhex-decode-region-external): Remove always-nil
var `firstline`.

* lisp/mail/emacsbug.el: Use lexical-binding.
(report-emacs-bug): Remove always-nil var `message-end-point`.

* lisp/mail/rmail-spam-filter.el: Use lexical-binding.
(bbdb/mail_auto_create_p): Declare variable.

* lisp/mail/rmail.el (rmail-get-new-mail): Remove always-nil var
`delete-files`.

* lisp/mail/rmailout.el: Use lexical-binding.
(rmail-output-read-file-name): Remove unused var `err`.
(rmail-convert-to-babyl-format): Remove unused var `count`.
(rmail-output-as-mbox): Remove unused vars `from` and `date`.

* lisp/mail/rmailsort.el: Use lexical-binding.
(rmail-sort-messages): Remove unused var `msginfo`.

* lisp/mail/rfc822.el: Use lexical-binding.
* lisp/mail/rmailedit.el: Use lexical-binding.
* lisp/mail/mailclient.el: Use lexical-binding.
* lisp/mail/blessmail.el: Use lexical-binding.
* lisp/mail/mail-hist.el: Use lexical-binding.
* lisp/mail/rmailkwd.el: Use lexical-binding.
* lisp/mail/rmailmsc.el: Use lexical-binding.
* lisp/mail/uce.el: Use lexical-binding.
* lisp/mail/unrmail.el: Use lexical-binding.