Add lua object references

Whenever a lua script references a core struct obj, increment a counter
in the obj struct. Core code will not free the obj, if there are any
lua references pointing to it, just makes it free-floating.
When lua script ends, the lua gc will free the free-floating objects.

Also exposes u.inventory to lua.

Breaks save and bones compat.

src/restore.c

@@ -201,6 +201,7 @@ struct obj *otmp;
     if (nhfp->structlevel)
         mread(nhfp->fd, (genericptr_t) otmp, sizeof(struct obj));
 
+    otmp->lua_ref_cnt = 0;
     /* next object pointers are invalid; otmp->cobj needs to be left
        as is--being non-null is key to restoring container contents */
     otmp->nobj = otmp->nexthere = (struct obj *) 0;