fix iolib.os on Darwin

 * PARSE-NAME-AND-OPTIONS used to return NIL as the foreign name if
   both a lisp name and a foreign name were specified, but the foreign
   name was unknown. In contrast if just a single foreign name was
   given, it would be returned without checking for availability.

   CFFI in turn would then loop in CFFI:FOREIGN-NAME, trying to deal
   with NIL.

   So: prefer known foreign names, but return one even if it is
   unknown.

 * Not all the DEFSYSCALL functions are available on Darwin, so this
   broke the build.