Add support for user-specified chunk allocators/deallocators.

Add new mallctl endpoints "arena<i>.chunk.alloc" and
"arena<i>.chunk.dealloc" to allow userspace to configure
jemalloc's chunk allocator and deallocator on a per-arena
basis.

include/jemalloc/jemalloc_protos.h.in

@@ -44,3 +44,6 @@ JEMALLOC_EXPORT void *	@je_@memalign(size_t alignment, size_t size)
 #ifdef JEMALLOC_OVERRIDE_VALLOC
 JEMALLOC_EXPORT void *	@je_@valloc(size_t size) JEMALLOC_ATTR(malloc);
 #endif
+
+typedef void *(chunk_alloc_t)(size_t, size_t, bool *, unsigned);
+typedef bool (chunk_dealloc_t)(void *, size_t, unsigned);