ziflist: typedef ziflist_t directly to zlist_t to avoid ugly casts

Problem: Keeping struct _ziflist_t abstract and casting it from/to ziflist_t is an ugly hack
Solution: typedef ziflist_t directly to zlist_t in czmq.h so no casts are required

This looses a bit of type safety since now one could potentially pass another zlist_t as argument to ziflist_t methods by accident. But you notice that quickly enough that I don't think it's a problem.