don't take over M-x shell started for unrelated buffers

[積丹尼 Dan Jacobson]

Once one makes a tramp connection, any subsequent use of M-x shell is
altered, **even when invoked from innocent unrelated buffers**.

It now asks "Remote shell path: ...".

Well M-x shell's behavior is altered, but its docstring remains the
same.

So nobody can figure out how to just get it to act like it used to,
without restarting Emacs.

tramp-version "2.3.2"