[libc] Add new libc

This new libc is mostly from scratch, with *printf() functions provided
by Marco Paland and Eyal Rozenberg's tiny printf library, and malloc and
friends provided by dlmalloc.

src/libraries/libc/string/strncat.cpp

@@ -0,0 +1,27 @@
+/** \file strncat.cpp
+  *
+  * This file is part of the C standard library for the jsix operating
+  * system.
+  *
+  * This Source Code Form is subject to the terms of the Mozilla Public
+  * License, v. 2.0. If a copy of the MPL was not distributed with this
+  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+  */
+
+#include <string.h>
+#include <stddef.h>
+#include <__j6libc/copy.h>
+
+char *strncat(char * restrict s1, const char * restrict s2, size_t n) {
+    if (!s1 || !s2 || !n)
+        return s1;
+
+    char *dest = s1;
+    char const *src = s2;
+    while (*dest++);
+    while (*src && n--) *dest++ = *src++;
+    *dest = 0;
+
+    return s1;
+}
+