Change the default stack size to safe values on OpenBSD.

With this patch racket will never grow beyond of the defaults limits of
the OS and also it doesn't limit the stack size to a fixed value.

Merge to v5.3.4

src/racket/sconfig.h

@@ -313,15 +313,21 @@
 #  define SCHEME_PLATFORM_LIBRARY_SUBPATH "i386-openbsd"
 # endif
 
-/* Same reason as for FreeBSD? */
-# define ASSUME_FIXED_STACK_SIZE
-# define FIXED_STACK_SIZE 1048576
+# include <sys/param.h>
+# if OpenBSD < 201211
+/* This is needed for (pre-5.2) userspace threads: */
+#  define ASSUME_FIXED_STACK_SIZE
+#  define FIXED_STACK_SIZE 1048576
+# endif
 
 # include "uconfig.h"
 # undef HAS_STANDARD_IOB
-
 # define HAS_BSD_IOB
 
+/* Default UNIX_STACK_MAXIMUM is too big for a non-root user. */
+# undef UNIX_STACK_MAXIMUM
+# define UNIX_STACK_MAXIMUM 4194304
+
 #ifndef __ELF__
 # define UNDERSCORE_DYNLOAD_SYMBOL_PREFIX
 #endif