Sitting on the floor scuffs engravings

Reimplementation of a pull request for the same feature; this scuffs the
engraving before most of the special cases that do involve sitting
directly on the surface happen, rather than as a special case that
happens if none of the others do (otherwise, sitting on an object or
laying an egg or various other things would leave the engraving intact.)

doc/xnh-changelog-9.0.md

@@ -49,6 +49,7 @@ changes:
   least 25%.
 - The potion of gain energy can be alchemized by combining the potions of full
   healing and gain ability.
+- Sitting down can partly wipe engravings on your space.
 
 ### Interface changes
 

src/sit.c

@@ -273,6 +273,7 @@ dosit(void)
         goto in_water;
     }
 
+    u_wipe_engr(rnd(5));
     if (OBJ_AT(u.ux, u.uy)
         /* ensure we're not standing on the precipice */
         && !(uteetering_at_seen_pit(trap) || uescaped_shaft(trap))) {