Fix stuck travel for good

My fixes to the travel stuck oscillation did not fix all of them,
and I've even seen a 3-step loop - which my fixes cannot detect.
I guess there could be arbitrary-sized loops too.

To definitely fix this, keep track of all the map locations travel
has moved the hero through, and if it tries to go on a location already
used, stop travel and give the unsure -message.

include/decl.h

@@ -938,6 +938,7 @@ struct instance_globals {
     /* hack.c */
     anything tmp_anything;
     int wc; /* current weight_cap(); valid after call to inv_weight() */
+    struct selectionvar *travelmap;
 
     /* insight.c */