Add new getentropy() system call. Code and pressure from matthew.

I accepted that he's right (again) to seperate this out from heavy
sysctl API and this will simply a variety of things.  Functionname
is not used by anyone in the ports tree, so we guess we can use it.
Shocking that no application has a function called this.
ok matthew & others who pushed him to start this early on

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.86 2014/05/16 21:28:15 tedu Exp $ */
+/*	$OpenBSD: unistd.h,v 1.87 2014/06/13 08:26:10 deraadt Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -538,6 +538,7 @@ void	 setusershell(void);
 int	 strtofflags(char **, u_int32_t *, u_int32_t *);
 int	 swapctl(int cmd, const void *arg, int misc);
 int	 syscall(int, ...);
+ssize_t	 getentropy(void *, size_t);
 pid_t	 __tfork_thread(const struct __tfork *, size_t, void (*)(void *),
 	    void *);
 #endif /* __BSD_VISIBLE */