Add commentary for last change.

Add commentary for last change.
(XMALLOC_PUT_SIZE, XMALLOC_GET_SIZE): New macros to handle
sizeof(size_t) != 4.
(overrun_check_malloc, overrun_check_realloc, overrun_check_free):
Use them.  Also clear header and trailer of freed memory.
(GC_STRING_OVERRUN_COOKIE_SIZE): Rename from GC_STRING_EXTRA.
(string_overrun_cookie): Rename from string_overrun_pattern.
(GC_STRING_EXTRA): Define from GC_STRING_OVERRUN_COOKIE_SIZE.