Add OptReuseAddr to example and handle closing

lib/pure/asyncnet.nim

@@ -36,12 +36,14 @@
 ##   proc processClient(client: AsyncSocket) {.async.} =
 ##     while true:
 ##       let line = await client.recvLine()
+##       if line.len == 0: break
 ##       for c in clients:
 ##         await c.send(line & "\c\L")
 ##
 ##   proc serve() {.async.} =
 ##     clients = @[]
 ##     var server = newAsyncSocket()
+##     server.setSockOpt(OptReuseAddr, true)
 ##     server.bindAddr(Port(12345))
 ##     server.listen()
 ##