* net/tramp.el (top): Require 'shell. Use `tramp-unload-hook' but

* net/tramp.el (top): Require 'shell.  Use `tramp-unload-hook' but
`tramp-cache-unload-hook' where appropriate.
(tramp-methods): Rename `tramp-remote-sh' to
`tramp-remote-shell'.  Add `tramp-remote-shell-args'.
(tramp-handle-shell-command): New defun, moved from tramp-sh.el.

* net/tramp-sh.el (top): Don't require 'shell.
(tramp-methods): Add `tramp-remote-shell' and
`tramp-remote-shell-args' entries.
(tramp-sh-file-name-handler-alist): Use `tramp-handle-shell-command'.
(tramp-sh-handle-shell-command): Remove.
(tramp-find-shell, tramp-open-connection-setup-interactive-shell):
Use `tramp-remote-shell'.