Implement INHERIT_ZERO for minherit(2).

INHERIT_ZERO is an OpenBSD feature.

When a page is marked as such, it would be zeroed
upon fork().

This would be used in new arc4random(3) functions.

PR:	182610
Reviewed by:	kib (earlier version)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D427

newlib/libc/sys/rtems/include/sys/mman.h

@@ -43,6 +43,7 @@
 #define INHERIT_SHARE	0
 #define INHERIT_COPY	1
 #define INHERIT_NONE	2
+#define INHERIT_ZERO	3
 #endif
 
 /*