trunk only: preserving context (src files)

Pat Rankin wrote:
> collect them all into some new struct and
> save that separately rather than jamming more non-option stuff
> into struct flags.

This patch:
- collects all context/tracking related fields from flags
  into a new structure called "context."
It also adds the following to the new structure:
- stethoscope turn support
- victual support
- tin support

src/region.c

@@ -852,7 +852,7 @@ int radius, ttl;
 	tmprect.hy--;
     }
     ff->ttl = ttl;
-    if (!in_mklev && !flags.mon_moving)
+    if (!in_mklev && !context.mon_moving)
 	set_heros_fault(ff);		/* assume player has created it */
  /* ff->can_enter_f = enter_force_field; */
  /* ff->can_leave_f = enter_force_field; */
@@ -971,7 +971,7 @@ int damage;
 	tmprect.hy--;
     }
     cloud->ttl = rn1(3,4);
-    if (!in_mklev && !flags.mon_moving)
+    if (!in_mklev && !context.mon_moving)
 	set_heros_fault(cloud);		/* assume player has created it */
     cloud->inside_f = INSIDE_GAS_CLOUD;
     cloud->expire_f = EXPIRE_GAS_CLOUD;