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/read.c

@@ -1065,7 +1065,7 @@ register struct obj	*sobj;
 			else
 			    u.uen = (u.uenmax += d(5,4));
 		    }
-		    flags.botl = 1;
+		    context.botl = 1;
 		    break;
 		}
 		known = TRUE;