bug#3767: 23.0.94; Functions in password-cache should be autoloaded

I was wondering why jabber.el didn't cache my password, even though I
set password-cache and password-cache-expiry correctly, and found the
reason: jabber.el checks whether the running Emacs is new enough to have
password-cache.el by (fboundp 'password-read), but this function is not
autoloaded.  The problem is masked if you use Gnus.

Thus I propose that at least `password-read' be autoloaded, possibly
others as well.

(Accusations of me being a misguided elisp programmer are welcome,
especially with suggestions of better ways to check for this feature.)

Maguns