TCP_NODELAY is not supported

[ValdikSS]

As of 1.2.1, TCP_NODELAY socket option (--tcp-nodelay or --socket-flags TCP_NODELAY) doesn't do anything, silently.
OpenVPN skips is in case of DCO-win, and there's no API to set it in the kernel driver.

https://github.com/OpenVPN/openvpn/blob/a7f80d402fb95df3c58a8fc5d12cdb8f39c37d3e/src/openvpn/socket.c#L957-L961

static bool
socket_set_flags(socket_descriptor_t sd, unsigned int sockflags)
{
    /* SF_TCP_NODELAY doesn't make sense for dco-win */
    if ((sockflags & SF_TCP_NODELAY) && (!(sockflags & SF_DCO_WIN)))

[lstipakov]

@cron2 Shall we just enable it by default?