Bring in compatibility glue for libsodium

The idea is untouched upstream sources live in sys/contrib/libsodium.

sys/crypto/libsodium are support routines or compatibility headers to allow
building unmodified upstream code.

This is not yet integrated into the build system, so no functional change.

sys/crypto/libsodium/limits.h

@@ -0,0 +1,3 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */
+#include <sys/limits.h>

sys/crypto/libsodium/randombytes.c

@@ -0,0 +1,13 @@
+/* This file is in the public domain. */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+#include <sys/libkern.h>
+
+#include <sodium/randombytes.h>
+
+void
+randombytes_buf(void *buf, size_t size)
+{
+	arc4random_buf(buf, size);
+}

sys/crypto/libsodium/stddef.h

@@ -0,0 +1,3 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */
+#include <sys/types.h>

sys/crypto/libsodium/stdint.h

@@ -0,0 +1,3 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */
+#include <sys/stdint.h>

sys/crypto/libsodium/stdio.h

@@ -0,0 +1,2 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */

sys/crypto/libsodium/stdlib.h

@@ -0,0 +1,2 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */

sys/crypto/libsodium/string.h

@@ -0,0 +1,2 @@
+/* This file is in the public domain */
+/* $FreeBSD$ */

sys/crypto/libsodium/utils.c

@@ -0,0 +1,14 @@
+/* This file is in the public domain. */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+#include <sys/types.h>
+#include <sys/systm.h>
+
+#include <sodium/utils.h>
+
+void
+sodium_memzero(void *b, size_t n)
+{
+	explicit_bzero(b, n);
+}