Move the _atfork_list definition to atexit.c so that the fork syscall stub

doesn't get pulled into all static executables
ok millert@ jca@

src/lib/libc/stdlib/atexit.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: atexit.c,v 1.22 2015/10/25 18:01:24 guenther Exp $ */
+/*	$OpenBSD: atexit.c,v 1.23 2015/10/25 18:03:17 guenther Exp $ */
 /*
  * Copyright (c) 2002 Daniel Hartmeier
  * All rights reserved.
@@ -41,6 +41,10 @@
 struct atexit *__atexit;
 static int restartloop;
 
+/* define and initialize the list */
+struct atfork_listhead _atfork_list = TAILQ_HEAD_INITIALIZER(_atfork_list);
+
+
 /*
  * Function pointers are stored in a linked list of pages. The list
  * is initially empty, and pages are allocated on demand. The first