Add direct remote copying in Tramp's scp support

Add direct remote copying in Tramp's scp support

* doc/misc/tramp.texi: Use @trampfn{} function where possible.
(Top, Configuration): Insert section 'Ssh setup' and remove
section 'Windows setup hints' in menu.
(Default Method): Mention tramp-use-scp-direct-remote-copying.
(Ssh setup): New node.
(Windows setup hints): Move it to that new node.
(Frequently Asked Questions): Move items about ssh config to that node.

* etc/NEWS: Add Tramp's direct remote copying feature.  Fix typos.

* lisp/net/tramp-sh.el (tramp-use-scp-direct-remote-copying):
New defcustom.
(tramp-methods) <scp, scpx>: Add "%y" marker.
(tramp-scp-direct-remote-copying): New defun.
(tramp-do-copy-or-rename-file-out-of-band): Extend for direct
remote copying.

* lisp/net/tramp.el (tramp-methods): Extend docstring.
(tramp-password-prompt-not-unique): New defvar.
(tramp-read-passwd): Adapt docstring.
(tramp-read-passwd-without-cache): New defun.
(tramp-action-password): Call it.