add uv udp try send

Please review the commits and give me comments on them.
quote by bnoordhuis from last thread

> uv_try_write() returns the number of bytes written. That would be kind of superfluous for uv_udp_try_send() because datagram writes are atomic: everything is written or nothing is but partial writes don't happen. A boolean is probably sufficient but then again, returning the number of bytes written doesn't actually hurt and is "bool-y" to