2075: Add ControlMessage::Ipv6HopLimit

When sending IPv6 packets with `sendmsg`, Linux does not use the hop limit
set on the socket. Instead, the hop limit has to be specified for each
individual message with ancillary data in a cmsg.

This commit adds the enum variant
`ControlMessage::Ipv6HopLimit` to specify the limit. The variant is
available on the `net` feature flag for Linux, MacOs, FreeBSD,
DragonflyBSD, Android, iOS and Haiku.

Author: sgasse