Locate password icon in global-mode-string

Locate password icon in global-mode-string

* doc/emacs/mini.texi (Passwords): Precise the location of the
password icon.

* doc/lispref/minibuf.texi (Reading a Password): The password icon
is added to global-mode-string.

* lisp/auth-source.el (read-passwd--mode-line-buffer): Remove.
(read-passwd--hide-password): Fix docstring.
(read-passwd-toggle-visibility): Don't use
`read-passwd--mode-line-buffer'.  Check for `read-passwd-mode'.
Force update in all mode lines.
(read-passwd-mode): Set `read-passwd--mode-line-icon' in
`global-mode-string'.  (Bug#73768)