Add 'pickup_stolen' option

Add pickup_stolen option to autopick items stolen from you by a nymph or
monkey, even if they don't match your normal autopickup settings.
Replace was_dropped, was_thrown with a 2-bit bitfield that can contain
values LOST_DROPPED, LOST_THROWN, and LOST_STOLEN (or 0), since they
should all be mutually exclusive anyway as they track the most recent
way the item left the hero's inventory.

[Rebase/merge conflict fixed up.  PR]

src/steal.c

@@ -527,6 +527,7 @@ steal(struct monst* mtmp, char* objnambuf)
     (void) encumber_msg();
     could_petrify = (otmp->otyp == CORPSE
                      && touch_petrifies(&mons[otmp->corpsenm]));
+    otmp->how_lost = LOST_STOLEN;
     (void) mpickobj(mtmp, otmp); /* may free otmp */
     if (could_petrify && !(mtmp->misc_worn_check & W_ARMG)) {
         minstapetrify(mtmp, TRUE);