Add `auth-info-password' and use it instead of ad hoc code

* lisp/auth-source.el (auth-info-password):
  Extract from `auth-source-pick-first-password'.
(auth-source-pick-first-password, auth-source-secrets-create)
(auth-source-user-and-password): Use `auth-info-password'.
* lisp/erc/erc-services.el (erc-nickserv-get-password):
  Use `auth-source-pick-first-password'.
* lisp/erc/erc.el (erc-open, erc-server-join-channel): Likewise.
* lisp/gnus/mail-source.el (mail-source-set-1): Add a comment.
* lisp/gnus/nnimap.el (nnimap-credentials): Use `auth-info-password'.
* lisp/gnus/nntp.el (nntp-send-authinfo): Likewise.
* lisp/mail/rmail.el (rmail-get-remote-password): Likewise.
* lisp/mail/smtpmail.el (smtpmail-try-auth-methods): Likewise.
* lisp/net/sieve-manage.el (sieve-sasl-auth): Likewise.
* lisp/net/tramp.el (tramp-read-passwd): Likewise.
* lisp/net/rcirc.el (rcirc): Likewise (fixes a bug: the possibility
  that password might be a function was not handled).

lisp/mail/smtpmail.el

@@ -554,11 +554,9 @@ for `smtpmail-try-auth-method'.")
 		      :create ask-for-password)))
          (mech (or (plist-get auth-info :smtp-auth) (car mechs)))
          (user (plist-get auth-info :user))
-         (password (plist-get auth-info :secret))
+         (password (auth-info-password auth-info))
 	 (save-function (and ask-for-password
 			     (plist-get auth-info :save-function))))
-    (when (functionp password)
-      (setq password (funcall password)))
     (when (and user
 	       (not password))
       ;; The user has stored the user name, but not the password, so
@@ -573,9 +571,7 @@ for `smtpmail-try-auth-method'.")
 	      :user smtpmail-smtp-user
 	      :require '(:user :secret)
 	      :create t))
-	    password (plist-get auth-info :secret)))
-    (when (functionp password)
-      (setq password (funcall password)))
+	    password (auth-info-password auth-info)))
     (let ((result (catch 'done
                     (if (and mech user password)
 		        (smtpmail-try-auth-method process mech user password)