Export getentropy and getrandom calls

getentropy per OpenBSD

  http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2

getrandom per Linux

  http://man7.org/linux/man-pages/man2/getrandom.2.html

  Note that GRND_NONBLOCK is not handled

winsup/cygwin/include/sys/random.h

@@ -0,0 +1,29 @@
+/* sys/random.h header file for Cygwin.
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for details. */
+
+#ifndef _SYS_RANDOM_H
+#define _SYS_RANDOM_H
+
+#include <_ansi.h>
+#include <sys/types.h>
+
+/* getrandom flags */
+#define GRND_NONBLOCK	1
+#define GRND_RANDOM	2
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+ssize_t getrandom (void *__ptr, size_t __len, unsigned int __flags);
+int getentropy (void *__ptr, size_t __len);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _SYS_RANDOM_H */