rmail.el: Prefer #' to quote function names

rmail.el: Prefer #' to quote function names

* lisp/mail/rmail.el (rmail-pop-to-buffer, rmail-mode-map)
(rmail-mode-1, rmail-generate-viewer-buffer, rmail-variables)
(rmail-find-all-files, rmail-insert-inbox-text)
(rmail-set-message-counters, rmail-only-expunge, rmail-reply)
(rmail-resend, rmail-fontify-buffer-function)
(rmail-unfontify-buffer-function, rmail-install-speedbar-variables)
(after-save-hook): Use #' where applicable.

(cherry picked from commit 048eaadd8cc97faf0f3e70a8d81d06f915c52081)