To see this in action:

   $ touch foo
   $ emacs -Q
   M-x eshell
   cp foo bar -s
   ls -l bar

`bar' should be a symlink pointing to `foo', but it's just a regular file.

This happens because, when :preserve-args is true, 
`eshell-eval-using-options' doesn't flatten and stringify the args to be 
parsed. That results in the various helper functions modifying the list 
of raw args in-place. When :external is set, if `eshell--process-option' 
finds an option it doesn't recognize, it throws, telling 
`eshell--do-opts' to run the external command instead. That requires 
having the original args, un-tampered with, in order to forward on to 
the external command.

Attached is a patch that fixes this by copying the raw args list for the 
:preserve-args case before manipulating it. I considered only adding 
this when :external is also set, but I think it's useful for callers to 
be able to inspect the raw args afterwards. This is used by `eshell/su' 
to check for the presence of a "-" argument, since 
`eshell-eval-using-options' doesn't handle that. (It might be nice to 
fix that too, but I think it's still useful to keep the original raw 
args around unchanged. Improving "-" support could be done later.)

As part of this, I also added several unit tests for `eshell/su' and 
`eshell/sudo' to make sure I didn't break anything there; these should 
pass both before and after the rest of the patch. I also added tests to 
make sure the original arguments are preserved for calling the external 
command; these should fail before the patch, and pass after.

There's just one remaining issue that I'm not sure how to fix: since I 
updated the `eshell-eval-using-options' macro, any file that uses it 
needs to be recompiled to get the new version. What's the right way to 
tell the build system about this? I just ran `touch lisp/eshell/*.el' 
locally, but I'm sure there's a better way so that people can just run 
`make' after pulling this patch.