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/dbridge.c

@@ -404,7 +404,7 @@ int dest, how;
 #define mk_message(dest) ((dest & 1) ? "" : (char *)0)
 #define mk_corpse(dest)  ((dest & 2) ? AD_DGST : AD_PHYS)
 		/* if monsters are moving, one of them caused the destruction */
-		if (flags.mon_moving)
+		if (context.mon_moving)
 		    monkilled(etmp->emon, mk_message(dest), mk_corpse(dest));
 		else		/* you caused it */
 		    xkilled(etmp->emon, dest);