Use RemoteCommand option for Tramp's sshx and scpx methods

Use RemoteCommand option for Tramp's sshx and scpx methods

* doc/misc/tramp.texi (Inline methods) <sshx>:
(External methods) <scpx>: Adapt call sequence.
(Remote shell setup): Mention, that sshx and scpx overwrite
RemoteCommand.
(Remote processes): Restriction: direct asynchronous processes
cannot be used when RemoteCommand is in use.
`tramp-remote-process-environment' is not ignored any longer.

* lisp/net/tramp-sh.el (tramp-methods) <sshx, scpx>: Handle login
shell via RemoteCommand.  Remove `tramp-direct-async' parameter.
(tramp-maybe-open-connection): Add "-i" to login.

* lisp/net/tramp-smb.el (tramp-smb-errors):
Add "NT_STATUS_NOT_SUPPORTED".
(tramp-smb-handle-insert-directory): Fix point moving error.

* test/lisp/net/tramp-tests.el (tramp-test34-explicit-shell-file-name):
Use `get-buffer-process' where appropriate.