Add route lifetime from Router Advertisement

[ColinMcInnes]

Sends route lifetime to NETLINK RTA_EXPIRES

Partially addresses #428

Still need to send hoplimit

[rsmarples]

This looks nice! Can we also extract the lifetime in if_copyrt in if-linux.c please?

[ColinMcInnes]

This looks nice! Can we also extract the lifetime in if_copyrt in if-linux.c please?

Yes, I can do that.

[rsmarples]

Gah, and I forgot one last thing.

Before setting each rt_expires, check that the lifetime (before working out elapsed) is not ND6_INFINITE_LIFETIME.

[ColinMcInnes]

ND6_INFINITE_LIFETIME

Something like this?

		if (rap->lifetime != ND6_INFINITE_LIFETIME)
		{
			elapsed = (uint32_t)eloop_timespec_diff(&now, &rap->acquired, NULL);
			rt->rt_expires = rap->lifetime - elapsed;
		}
		else
			rt->rt_expires = ND6_INFINITE_LIFETIME;

[rsmarples]

Exactly! We have a lot of that code, so maybe we benefit from a helper function in common.c?

uint32_t
lifetime_left(uint32_t lifetime, const struct timespec *aquired, const struct timespec *now)
{
    uint32_t elpased;

    if (lifetime == INFINITE_LIFETIME)
        return lifetime;

    elapsed = (uint32_t)eloop_timespec_diff(now, &acquired, NULL);
    if (elapsed > lifetime)
        return 0;
    return lifetime - elapsed;
}

then we can simply write:

rt->rt_expires = lifetime_left(rap->lifetime, &rap->aquired, &now);

Thoughts? If you like it, #define INFINITE_LIFETIME (~0U) please in common.h
A future patch could then replace bits where we do this to reduce code even more.

[rsmarples]

Last round of changes, then I think we're good to merge!

[rsmarples]

The CI picked up a warning we need to address, but this is looking really good now.

[rsmarples]

@ColinMcInnes thanks for the excellent work here.

[rsmarples]

@ColinMcInnes I used the new function to reduce more code in 7b94a2e

Hopefully everything still works!