Enhancement - Handle v6 route HopLimit/TTL

[ColinMcInnes]

10.1.0

Similar to #428 dhcpcd does not pass on the IPV6 route hop limit/TTL to netlink, nor read it when copying an existing route.

Gate this within HAVE_ROUTE_TTL and linux-only. This feature will not trigger if noipv6rs is set as the kernel will handle this itself.

ip -6 route show default

default via <MAC> dev eth0 proto ra metric 1024 expires 1512sec hoplimit 64 pref medium

Similar to MTU, route should only need to be updated if this value changes from last time it was set.