lib/string/sprintf/, lib/, src/, tests/: Move all sprintf(3)-like APIs to a subdirectory

And have a separate file for each pair of APIs.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/string/sprintf/xasprintf.c

@@ -0,0 +1,14 @@
+// SPDX-FileCopyrightText: 2023-2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include <config.h>
+
+#include "string/sprintf/xasprintf.h"
+
+#include <stdarg.h>
+
+
+extern inline int xasprintf(char **restrict s, const char *restrict fmt, ...);
+extern inline int xvasprintf(char **restrict s, const char *restrict fmt,
+    va_list ap);