(emacs_blocked_malloc): Set __malloc_extra_blocks here.

(emacs_blocked_malloc): Set __malloc_extra_blocks here.
(malloc_hysteresis): New variable.
(init_alloc_once): Initialize malloc_hysteresis.
(buffer_memory_full): New function.

(refill_memory_reserve): New function.

(SPARE_MEMORY): New macro.
(emacs_blocked_free): If no spare_memory now, and enough free
space is available, get a new spare.
(__malloc_size_t, _bytes_used): Declared.
(bytes_used_when_full): New variable.

(syms_of_alloc): Improve memory exhausted error message.
(init_alloc_once): Allocate spare_memory.
(memory_full): Free spare_memory.