trunk only: preserving context (include 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

include/display.h

@@ -42,7 +42,7 @@
 
 #define mon_warning(mon) (Warning && !(mon)->mpeaceful && 				\
 			 (distu((mon)->mx, (mon)->my) < 100) &&				\
-			 (((int) ((mon)->m_lev / 4)) >= flags.warnlevel))
+			 (((int) ((mon)->m_lev / 4)) >= context.warnlevel))
 
 /*
  * mon_visible()