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.

src/decl.c

@@ -422,8 +422,9 @@ const struct instance_globals g_init = {
 
 
     /* hack.c */
-    UNDEFINED_VALUES,
-    UNDEFINED_VALUE,
+    UNDEFINED_VALUES, /* tmp_anything */
+    UNDEFINED_VALUE, /* wc */
+    NULL, /* travelmap */
 
     /* invent.c */
     51, /* lastinvr */