#define _MAX_PAGE_SHIFT in MD _types.h as the maximum pagesize an arch

needs (looking at you sgi, but others required this before).  This is for
the circumstances we need pagesize known at compile time, not getpagesize()
runtime. Use it for malloc storage sizes, for shm, and to set pthread stack
default sizes.  The stack sizes were a mess, and pushing them towards
page-aligned is healthy move (which will also be needed by the coming
stack register checker)
ok guenther kettenis, discussion with stefan

src/include/pthread.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: pthread.h,v 1.3 2017/11/04 22:53:57 jca Exp $	*/
+/*	$OpenBSD: pthread.h,v 1.4 2018/03/05 01:15:26 deraadt Exp $	*/
 
 /*
  * Copyright (c) 1993, 1994 by Chris Provenzano, proven@mit.edu
@@ -51,7 +51,7 @@
  */
 #define PTHREAD_DESTRUCTOR_ITERATIONS		4
 #define PTHREAD_KEYS_MAX			256
-#define PTHREAD_STACK_MIN			2048
+#define PTHREAD_STACK_MIN			(1U << _MAX_PAGE_SHIFT)
 #define PTHREAD_THREADS_MAX			ULONG_MAX
 
 /*