for mmap objcodes, store the fd in the third word, as a scheme int

for mmap objcodes, store the fd in the third word, as a scheme int

This leaves space for native code.

* libguile/objcodes.h (SCM_OBJCODE_NATIVE_CODE)
  (SCM_SET_OBJCODE_NATIVE_CODE): Reserve the fourth word of objcode for
  "native code", whatever that means.

* libguile/objcodes.c: Update a comment.
  (make_objcode_by_mmap): Put the fd in the third word.