libc: io: implement file io buffering and internal locking for concurrency

lib/libc/io/stdio/stdin.c

@@ -0,0 +1,14 @@
+#include "file.h"
+
+#include <stdio.h>
+
+static struct __opaque_file __stdin = {
+	.f_fd = 0,
+	.f_flags = FILE_STATIC,
+	.f_buffer_mode = _IOLBF,
+};
+
+extern struct __opaque_file *__libc_file_stdin(void)
+{
+	return &__stdin;
+}