TTY: Add compile-time option for tile data hinting

Several people have asked if 3.6.0 supports playing with tiles on
a public server. Because there's no way for the user's end to know
what that white @ is, this adds special console escape codes for
tile hinting.

The idea was originally a patch called TelnetTiles by Justin Hiltscher,
but this expanded version comes via NAO, where it's been in use for years.

This is basically an interim hack, which should go away when/if
we support actual client-server model.

include/flag.h

@@ -259,6 +259,9 @@ struct instance_flags {
 #ifdef LAN_FEATURES
     boolean lan_mail;         /* mail is initialized */
     boolean lan_mail_fetched; /* mail is awaiting display */
+#endif
+#ifdef TTY_TILES_ESCCODES
+    boolean vt_tiledata;     /* output console codes for tile support in TTY */
 #endif
     boolean wizweight;        /* display weight of everything in wizard mode */
     /*