Make monsters not scared of the Sokoban rolling boulder traps

They aren't initially loaded with boulders, thus aren't dangerous.

This is particularly relevant for pets, who would otherwise be very
reluctant to follow their owners up to the level above.

doc/fixes5-0-1.txt

@@ -56,6 +56,7 @@ if the hero has already been told that an engraving vanished, ensure that
 aklys path shouldn't be shown for locations that the hero cannot see
 keep the player better-informed about status of the tether on their weapon
 don't overflow svp.plname on restore (pr #1570 by chasonr)
+monsters (including pets) aren't scared of the Sokoban rolling boulder traps
 
 
 Platform- and/or Interface-Specific Fixes

src/trap.c

@@ -1129,6 +1129,9 @@ m_harmless_trap(struct monst *mtmp, struct trap *ttmp)
     case LANDMINE:
         break;
     case ROLLING_BOULDER_TRAP:
+        /* the Sokoban rolling boulder traps are not dangerous */
+        if (In_sokoban(&u.uz))
+            return TRUE;
         break;
     case SLP_GAS_TRAP:
         if (resists_sleep(mtmp) || defended(mtmp, AD_SLEE))