If stdin in the client is enable immediately, tmux will eat anything

sent to stdin before it is needed, which can be inconvenient (eg pasting
commands). Instead, start with stdin disabled and reuse MSG_STDIN from
server->client to mean that stdin should be enabled. Based on a diff
from Chris Johnsen.

server-fn.c

@@ -581,5 +581,8 @@ server_set_stdin_callback(struct client *c, void (*cb)(struct client *, int,
 
 	if (c->stdin_closed)
 		c->stdin_callback (c, 1, c->stdin_callback_data);
+
+	server_write_client(c, MSG_STDIN, NULL, 0);
+
 	return (0);
 }