starting pet identification

Make it possible to identify your starting pet throughout the game
via is_starting_pet(mon) macro.

include/context.h

@@ -72,6 +72,7 @@ struct context_info {
 	unsigned run;		/* 0: h (etc), 1: H (etc), 2: fh (etc) */
 				/* 3: FH, 4: ff+, 5: ff-, 6: FF+, 7: FF- */
 				/* 8: travel */
+	unsigned startingpet_mid;
 	int	 warnlevel;
 	int	 djinni_count, ghost_count;	/* potion effect tuning */
 	long stethoscope_move;