Some Tramp methods allow to change the remote login shell

Some Tramp methods allow to change the remote login shell

* doc/misc/tramp.texi (Inline methods) <sshx, plink, plinkx>:
(External methods) <scpx, pscp, psftp>: Mention, that the remote
login shell could be changed.
(Remote shell setup): Remove description of properties
"remote-shell-login" and "remote-shell-args", they don't matter
here.  Changing the default remote shell works only for some
methods.
(Frequently Asked Questions): Refer to alternative approach fixing
zsh problems.

* etc/NEWS: Some Tramp methods allow to change the remote login shell.

* lisp/net/tramp-sh.el (tramp-default-remote-shell): New defconst.
(tramp-methods): Use it.
(tramp-get-sh-extra-args): New defun.
(tramp-open-shell, tramp-maybe-open-connection): Use it.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.