Update route if lifetime needs changing (#441)

Update route if lifetime needs changing (#441)

Everything about IPv6 is lifetime, not expiry.
Only linux routes have an expiry.

OK, this is not consistent at all!
In the kernel RTA_EXPIRES is only set for IPv4 multicast routes
when emitting route changes and only read for IPv6 routes when
userland makes route changes.
We cannot set this for non IPv6 routes currently.

To make it worse, we set a UINT32 for IPv6 routes but read a
UINT64 for IPv4 multicast routes.

To make this even more totally bonkers, the expiry we set for
the IPv6 route can be read back via RTA_CACHEINE rta_expires
but we need to convert to divide it by hz.

Now we can read kernel route lifetime correctly, we can compare this to what we think it should be when building the routing table (allowing for some deviation due to processing time) and update if needed.

Fixes #428.