Here's a quick stab at it.  There's some cargo-cult programming in
there, and I have to go over the error cases again to make sure there's
no FD leaks, but it'll probably look something like the patch included.

Usage is:

(call-process "echo" nil '(:file "/tmp/hello") nil "thing")

or

(call-process "echo" nil '((:file "/tmp/hello") "/tmp/error") nil "thing")

So with this, `call-process' can do all the combinations of
STDERR/STDOUT to file/buffers, except that you can't put STDERR in one
buffer and STDOUT in a different buffer.

But you can get STDERR in a buffer by itself, and STDOUT in a file, so
it's progress of a kind...