Update arc4random module from OpenBSD and LibreSSL

Rework arc4random_stir() and arc4random_addrandom() code over the new
internal API, and documentation in the man page. Adapt the code to the
local build system.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=85827

src/arc4random_openbsd.h

@@ -0,0 +1,61 @@
+/*	$OpenBSD: arc4random.h,v 1.3 2014/07/20 20:51:13 bcook Exp $	*/
+
+/*
+ * Copyright (c) 1996, David Mazieres <dm@uun.org>
+ * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
+ * Copyright (c) 2013, Markus Friedl <markus@openbsd.org>
+ * Copyright (c) 2014, Theo de Raadt <deraadt@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * Stub functions for portability.
+ */
+#include <sys/mman.h>
+
+#include <signal.h>
+
+#include "thread_private.h"
+
+static inline void
+_getentropy_fail(void)
+{
+	raise(SIGKILL);
+}
+
+static inline int
+_rs_allocate(struct _rs **rsp, struct _rsx **rsxp)
+{
+	struct {
+		struct _rs rs;
+		struct _rsx rsx;
+	} *p;
+
+	if ((p = mmap(NULL, sizeof(*p), PROT_READ|PROT_WRITE,
+	    MAP_ANON|MAP_PRIVATE, -1, 0)) == MAP_FAILED)
+		return (-1);
+	if (minherit(p, sizeof(*p), MAP_INHERIT_ZERO) == -1) {
+		munmap(p, sizeof(*p));
+		return (-1);
+	}
+
+	*rsp = &p->rs;
+	*rsxp = &p->rsx;
+	return (0);
+}
+
+static inline void
+_rs_forkdetect(void)
+{
+}