[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.h

@@ -0,0 +1,65 @@
+#pragma once
+/** \file string.h
+  * String handling
+  *
+  * 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 <__j6libc/null.h>
+#include <__j6libc/restrict.h>
+#include <__j6libc/size_t.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Copying functions
+//
+void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
+void *memmove(void * restrict s1, const void * restrict s2, size_t n);
+char *strncpy(char * restrict s1, const char * restrict s2, size_t n);
+
+// Concatenation functions
+//
+char *strncat(char * restrict s1, const char * restrict s2, size_t n);
+
+// Comparison functions
+//
+int memcmp(const void *s1, const void *s2, size_t n);
+int strcmp(const char *s1, const char *s2);
+int strcoll(const char *s1, const char *s2);
+int strncmp(const char *s1, const char *s2, size_t n);
+size_t strxfrm(char * restrict s1, const char * restrict s2, size_t n);
+
+// Search functions
+//
+void *memchr(const void *s, int c, size_t n);
+char *strchr(const char *s, int c);
+size_t strcspn(const char *s1, const char *s2);
+char *strpbrk(const char *s1, const char *s2);
+char *strrchr(const char *s, int c);
+size_t strspn(const char *s1, const char *s2);
+char *strstr(const char *s1, const char *s2);
+char *strtok(char * restrict s1, const char * restrict s2);
+
+// Miscellaneous functions
+//
+void *memset(void *s, int c, size_t n);
+char *strerror(int errnum);
+size_t strlen(const char *s);
+
+// Deprecated functions
+//
+char *strcpy(char * restrict, const char * restrict)
+    __attribute__((deprecated("strcpy is unsafe, do not use", "strncpy")));
+char *strcat(char * restrict, const char * restrict)
+    __attribute__((deprecated("strcat is unsafe, do not use", "strncat")));
+
+#ifdef __cplusplus
+} // extern "C"
+#endif