use safe_malloc and safe_calloc in generated code

use safe_malloc and safe_calloc in generated code

* src/generator.ml (safe_malloc): Define to guestfs_safe_malloc.
(safe_calloc): Define to guestfs_safe_calloc.
[most generated code]: Fail immediately upon failure of otherwise-
unchecked malloc and calloc calls.
* src/guestfs.c: Include <stddef.h>.
(xalloc_oversized): Define.
* src/guestfs.h (guestfs_safe_calloc): Declare.