ffi:defcalback isn't consitant between `SFFI` and `DFFI`

SFFI and DFFI implementation of ffi:defcallback differ – the first returns a list with one element – the foreign pointer, while the latter returns just the foreign pointer.

This behaviour should be unified (CFFI uses DFFI extensively, so it would be best to fix the special form in SFFI).

changed milestone to %ECL 16.2.0

steps to reproduce (in repl)

CL-USER> (defun xxx ()
           (ffi:defcallback foo :void () (print "hi")))
XXX
CL-USER> (xxx)
#<foreign :POINTER-VOID>
CL-USER> (compile 'xxx)
;;; OPTIMIZE levels: Safety=2, Space=0, Speed=3, Debug=3
;;;
;;; End of Pass 1.
XXX
NIL
NIL
CL-USER> (xxx)
(#<foreign :POINTER-VOID>)