Add Tramp method "nspawn"

Add Tramp method "nspawn"

* doc/misc/tramp.texi: Adapt some markups.
(Inline methods) <nspawn>: Add.
(Default Host): Mention default host for "nspawn".
(Customizing Methods): Remove tramp-nspawn.
(Password handling): Add "nspawn".

* etc/NEWS: New Tramp method "nspawn".

* lisp/net/tramp-compat.el (tramp-nspawn): Warn, if that package is used.

* lisp/net/tramp-container.el (tramp-nspawn-program): New defcustom.
(tramp-nspawn-method): New defconst.
(tramp-nspawn--completion-function, tramp-enable-nspawn-method):
New defuns.

* lisp/net/tramp.el (tramp-local-host-regexp): Add OpenSUSE host names.