Rewrite the code for reading and writing files. Now, if the client is

not attached, the server process asks it to open the file, similar to
how works for stdin, stdout, stderr. This makes special files like
/dev/fd/X work (used by some shells). stdin, stdout and stderr and
control mode are now just special cases of the same mechanism. This will
also make it easier to use for other commands that read files such as
source-file.

control-notify.c

@@ -54,7 +54,7 @@ control_notify_input(struct client *c, struct window_pane *wp,
 			else
 			    evbuffer_add_printf(message, "%c", buf[i]);
 		}
-		control_write_buffer(c, message);
+		control_write(c, "%s", EVBUFFER_DATA(message));
 		evbuffer_free(message);
 	}
 }