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

lib/libc/io/stdio/fopen.c

@@ -143,6 +143,12 @@ struct __opaque_file *fopen(const char *path, const char *mode)
 		return NULL;
 	}
 
+	if (fflags & FILE_BIN) {
+		out->f_buffer_mode = _IOFBF;
+	} else {
+		out->f_buffer_mode = _IOLBF;
+	}
+
 	out->f_fd = fd;
 	out->f_flags = fflags;
 	__set_errno(SUCCESS);