tramp: Tweak the ls-lisp declarations

tramp: Tweak the ls-lisp declarations

* lisp/net/tramp-sh.el (ls-lisp-use-insert-directory-program): Don't
declare its existence...
(tramp-sh-handle-insert-directory): ...test it instead.

* lisp/net/tramp.el (ls-lisp-dirs-first, ls-lisp-emulation)
(ls-lisp-ignore-case, ls-lisp-use-insert-directory-program)
(ls-lisp-verbosity): Move declaration...
(tramp-handle-insert-directory): ...to the point where we have a good
reason to think these variables exist.

(cherry picked from commit 45125e019c3698ff74ccb2183b789c25f9d3f574)