strlcpy的一个实现，因为strncpy有陷阱，为避免strncpy的陷阱，需要一些额外防御性编码。现在用strlcpy就方便很多了-an implementation of strlcpy, strncpy because there is a trap, in order to avoid the trap of strncpy, need some extra defensive coding. Strlcpy now a lot easier