Add caching through password-cache.

Add caching through password-cache.

* password-cache.el (password-cache-remove): Accept secrets that are
not strings.

* auth-source.el: Require `password-cache'.
(auth-source-hide-passwords, auth-source-cache): Remove and mark
obsolete.
(auth-source-magic): Marker for `password-cache' keys.
(auth-source-do-cache): Update docstring.
(auth-source-search): Use and check cache.
(auth-source-forget-all-cached, auth-source-remember)
(auth-source-recall, auth-source-forget, auth-source-forget+)
(auth-source-specmatchp): Caching support functions.
(auth-source-forget-user-or-password, auth-source-forget-all-cached):
Remove and obsolete.
(auth-source-user-or-password): Remove caching to further discourage
using it.  Always hide passwords.

* nnimap.el: Autoload `auth-source-forget+'.
(nnimap-open-connection-1): Use it if the connection fails.