Fix arena_prefork lock rank order for witness.

When witness is enabled, lock rank order needs to be preserved during
prefork, not only for each arena, but also across arenas. This change
breaks arena_prefork into further stages to ensure valid rank order
across arenas. Also changed test/unit/fork to use a manual arena to
catch this case.

test/unit/fork.c

@@ -9,6 +9,19 @@ TEST_BEGIN(test_fork) {
 	void *p;
 	pid_t pid;
 
+	/* Set up a manually managed arena for test. */
+	unsigned arena_ind;
+	size_t sz = sizeof(unsigned);
+	assert_d_eq(mallctl("arenas.create", (void *)&arena_ind, &sz, NULL, 0),
+	    0, "Unexpected mallctl() failure");
+
+	/* Migrate to the new arena. */
+	unsigned old_arena_ind;
+	sz = sizeof(old_arena_ind);
+	assert_d_eq(mallctl("thread.arena", (void *)&old_arena_ind, &sz,
+	    (void *)&arena_ind, sizeof(arena_ind)), 0,
+	    "Unexpected mallctl() failure");
+
 	p = malloc(1);
 	assert_ptr_not_null(p, "Unexpected malloc() failure");