Fallback to not using ECN if IP_TOS is not supported

[link2xt]

On Linux <3.13 sendmsg/sendmmsg system calls return EINVAL without sending anything if they encounter an IP_TOS cmsg. To ensure we can still send on such systems, remember if we got an EINVAL error and switch to "fallback mode" in which we do not try to transmit ECN at all and only use well-supported features.

[link2xt]

This is an alternative to #1512

[djc]

Seems reasonable to me!

[link2xt]

I tested, it works on my Android with old kernel 3.10.108.

[Ralith]

Thanks!