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/monmove.c

@@ -63,7 +63,7 @@ register struct monst *mtmp;
 		}
 	    } else if (is_digging()) {
 		/* chewing, wand/spell of digging are checked elsewhere */
-		watch_dig(mtmp, digging.pos.x, digging.pos.y, FALSE);
+		watch_dig(mtmp, context.digging.pos.x, context.digging.pos.y, FALSE);
 	    }
 	}
 }