Auto merge of #982 - aep:for-upstream, r=alexcrichton

define more TCP_ sockopts on bsd

seee #978

sources for sockopt numbers:
- openbsd: i can't find them, so maybe they dont exist.
- dragonfly: https://github.com/DragonFlyBSD/DragonFlyBSD/search?utf8=%E2%9C%93&q=TCP_KEEPINTVL&type=
- netbsd: https://github.com/IIJ-NetBSD/netbsd-src/search?utf8=%E2%9C%93&q=TCP_KEEPINTVL&type=