If we can identify the terminal as iTerm2 or as tmux, we can be sure

they support 256 and RGB colours, so set those flags too.

tmux.h

@@ -1171,6 +1171,7 @@ struct tty_term {
 #define TERM_NOXENL 0x2
 #define TERM_DECSLRM 0x4
 #define TERM_DECFRA 0x8
+#define TERM_RGBCOLOURS 0x10
 	int		 flags;
 
 	LIST_ENTRY(tty_term) entry;