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.

test/integration/chunk.c

@@ -0,0 +1,61 @@
+#include "test/jemalloc_test.h"
+
+chunk_alloc_t *old_alloc;
+chunk_dealloc_t *old_dealloc;
+
+bool
+chunk_dealloc(void *chunk, size_t size, unsigned arena_ind)
+{
+
+	return (old_dealloc(chunk, size, arena_ind));
+}
+
+void *
+chunk_alloc(size_t size, size_t alignment, bool *zero, unsigned arena_ind)
+{
+
+	return (old_alloc(size, alignment, zero, arena_ind));
+}
+
+TEST_BEGIN(test_chunk)
+{
+	void *p;
+	chunk_alloc_t *new_alloc;
+	chunk_dealloc_t *new_dealloc;
+	size_t old_size, new_size;
+
+	new_alloc = chunk_alloc;
+	new_dealloc = chunk_dealloc;
+	old_size = sizeof(chunk_alloc_t *);
+	new_size = sizeof(chunk_alloc_t *);
+
+	assert_d_eq(mallctl("arena.0.chunk.alloc", &old_alloc,
+	    &old_size, &new_alloc, new_size), 0,
+	    "Unexpected alloc error");
+	assert_ptr_ne(old_alloc, new_alloc,
+	    "Unexpected alloc error");
+	assert_d_eq(mallctl("arena.0.chunk.dealloc", &old_dealloc,
+	    &old_size, &new_dealloc, new_size), 0,
+	    "Unexpected dealloc error");
+	assert_ptr_ne(old_dealloc, new_dealloc,
+	    "Unexpected dealloc error");
+
+	p = mallocx(42, 0);
+	assert_ptr_ne(p, NULL, "Unexpected alloc error");
+	free(p);
+
+	assert_d_eq(mallctl("arena.0.chunk.alloc", NULL,
+	    NULL, &old_alloc, old_size), 0,
+	    "Unexpected alloc error");
+	assert_d_eq(mallctl("arena.0.chunk.dealloc", NULL,
+	    NULL, &old_dealloc, old_size), 0,
+	    "Unexpected dealloc error");
+}
+TEST_END
+
+int
+main(void)
+{
+
+	return (test(test_chunk));
+}