Prevent control clients from affecting the session size until they have

specified a size with refresh-client -C. Prompted by a different change
with the same purpose from George Nachman.

tmux.h

@@ -1345,6 +1345,7 @@ struct client {
 #define CLIENT_STATUSFORCE 0x80000
 #define CLIENT_DOUBLECLICK 0x100000
 #define CLIENT_TRIPLECLICK 0x200000
+#define CLIENT_SIZECHANGED 0x400000
 	int		 flags;
 	struct key_table *keytable;