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

lib/libc/io/stdio/fseek.c

@@ -0,0 +1,8 @@
+#include "file.h"
+
+#include <unistd.h>
+
+int fseek(struct __opaque_file *stream, long offset, int origin)
+{
+	return lseek(stream->f_fd, offset, origin);
+}