Let Tramp save passwords

Let Tramp save passwords

* lisp/auth-source.el (auth-source-secrets-saver): New defun.
(auth-source-secrets-create): Use it.

* lisp/net/secrets.el (secrets-struct-secret-content-type):
(secrets-create-item): Do not hard-code :xdg:schema.

* lisp/net/tramp.el (tramp-password-save-function): New defvar.
(tramp-read-passwd): Set it properly.
(tramp-process-actions):
* lisp/net/tramp-gvfs.el (tramp-gvfs-maybe-open-connection):
Save password.

* lisp/net/tramp-cmds.el (tramp-bug): Don't report
`tramp-password-save-function'.

* test/lisp/net/secrets-tests.el (secrets-test03-items):
Extend test with another :xdg:schema.