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/gnus/mail-source.el

@@ -454,7 +454,7 @@ the `mail-source-keyword-map' variable."
                                                   search))))
                     :user)))
              user-auth)
-            ((and
+            ((and               ; cf. 'auth-source-pick-first-password'
               (eq keyword :password)
               (setq pass-auth
                     (plist-get