Modernize and clean up gmalloc.c to assume C89 (Bug#9119).

Modernize and clean up gmalloc.c to assume C89 (Bug#9119).

* gmalloc.c: (_MALLOC_INTERNAL, _MALLOC_H, _PP, __ptr_t)
(__malloc_size_t, __malloc_ptrdiff_t):
Remove.  All uses removed, replaced by the definiens if needed,
since we can assume C89 or better now.
Include <stdint.h>, for PTRDIFF_MAX, uintptr_t.
(protect_malloc_state, align, get_contiguous_space)
(malloc_atfork_handler_prepare, malloc_atfork_handler_parent)
(malloc_atfork_handler_child, malloc_enable_thread)
(malloc_initialize_1, __malloc_initialize, morecore_nolock)
(_malloc_internal_nolock, _malloc_internal, malloc, _malloc)
(_free, _realloc, _free_internal_nolock, _free_internal, free, cfree)
(special_realloc, _realloc_internal_nolock, _realloc_internal)
(realloc, calloc, __default_morecore, memalign, valloc, checkhdr)
(freehook, mallochook, reallochook, mabort, mcheck, mprobe):
Define using prototypes, not old style.
(align, _malloc_internal_nolock, _free_internal_nolock, memalign):
Don't assume ptrdiff_t and uintptr_t are no wider than unsigned long.
(align): Don't assume that signed integer overflow wraps around.
Omit unused local var.
(malloc_initialize_1, morecore_nolock, _malloc_internal_nolock)
(_free_internal_nolock, memalign, mallochook, reallochook):
Omit no-longer-needed casts.
(valloc): Use getpagesize, not __getpagesize.
(MAGICWORD, MAGICFREE): Now randomish size_t values, not 32-bit.
(struct hdr): The 'magic' member is now size_t, not unsigned long.