Unveiling unveil(2).

This brings unveil into the tree, disabled by default - Currently
this will return EPERM on all attempts to use it until we are
fully certain it is ready for people to start using, but this
now allows for others to do more tweaking and experimentation.
Still needs to send the unveil's across forks and execs before
fully enabling.
Many thanks to robert@ and deraadt@ for extensive testing.
ok deraadt@

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.105 2017/12/12 01:12:34 deraadt Exp $ */
+/*	$OpenBSD: unistd.h,v 1.106 2018/07/13 09:25:22 beck Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -523,6 +523,7 @@ int	 swapctl(int cmd, const void *arg, int misc);
 int	 syscall(int, ...);
 int	 getentropy(void *, size_t);
 int	 pledge(const char *, const char *);
+int	 unveil(const char *, const char *);
 pid_t	 __tfork_thread(const struct __tfork *, size_t, void (*)(void *),
 	    void *);
 #endif /* __BSD_VISIBLE */