Make ZMQ_TOS work with IPv6 sockets

[saifhhasan]

Summary:
To set Type Of Service for IP layer packets ZMQ provides ZMQ_TOS socket
option. However this only works for v4 sockets. Considering things are moving to
IPv6 heavily (especially within enterprise networks), ZMQ should support setting
traffic class for v6 based on ZMQ_TOS.

There is a subtle difference between v4 and v6 in terms of the positioning of
field but TOS has same meaning in both v4 and v6. Linux provides following APIs
for v4/v6 to set TOS field value.

// For v4
setsockopt(fd, IPPROTO_IP, IP_TOS, tos, sizeof(tos));

// For v6
setsockopt(fd, IPPROTO_IPV6, IPV6_TCLASS, tos, sizeof(tos));

Test Plan:
Make sure Cmake works fine and all tests passes.
Imported this change to OpenR project and tested on our local testbed.
Captured some packets exchanged between PUB/SUB and ROUTER/ROUTER sockets
pairs. Verify that TCLASS value is set as per expectation.

Tasks: #2208

[bluca]

Thanks! Will it handle gracefully the case where IPv6 is not enabled? If not we can fix it up.

[bluca]

Also, for your next contributions, could you please follow the contributing guidelines for the git commit message?

Problem: X is broken

Solution: do Y and Z to fix X

http://zeromq.org/docs:contributing

[saifhhasan]

Thanks @bluca for quick response. Amazed by the speed of zeromq community :) I will follow the guide for future contributions (somehow missed that part).

I have not tested with IPv4 only system yet. Not sure what will be the behaviour (it might need masking off in case if IPv6 is not enabled).

[bluca]

Welcome to the community :-) What you've just seen in action is our C4.1 process: https://rfc.zeromq.org/spec:42/C4/ - IE. merge first, ask questions later :-)

I'll give it a go for the v4 case and fixup if there's a fallout, no worries.

[bluca]

ENOPROTOOPT is returned if IPv6 is not available, so it's a simple fix: #2210