make-string et al nulls memory if not given an initializer

make-string et al nulls memory if not given an initializer

* libguile/gc-malloc.c: Add a note that the gc-malloc does not clear the
  memory block, so users need to make sure it is initialized.

* libguile/bitvectors.c (scm_c_make_bitvector):
* libguile/bytevectors.c (scm_make_bytevector):
* libguile/strings.c (scm_c_make_string): If no initializer is given,
  initialize the bytes to 0. Prevents information leakage if an app uses
  make-string et al without initializers.

* libguile/foreign.c (make_cif): Initialize this too, to prevent leakage
  in the struct holes. Paranoia...