Prevent expiration of a v6 PD lease at t2 if DHCP server is unresponsive.

[wteiken]

Fixes issue #174.

The PD lease will still expire after restarting dhcpcd or if rebinding the lease with '-n'.
The former is WAI, the latter should be fixed in another patch.

[wteiken]

Not familiar enough with the inform model to know if INFORM/INFORMED should be added to the list of exceptions for RFC3633 Section 12 validation.

[perkelix]

That sudden drop of IPv6 at 3600 seconds is still an issue here. What I get from the ISP:

Jul 22 16:05:33 p8h61 dhcpcd[666]: dhcpcd-10.0.2 starting
Jul 22 16:05:33 p8h61 dhcpcd[674]: DUID 00:01:00:01:2b:cc:67:8e:c8:60:00:d0:73:4d
Jul 22 16:05:36 p8h61 dhcpcd[674]: enp4s0: waiting for carrier
Jul 22 16:05:38 p8h61 dhcpcd[674]: enp4s0: carrier acquired
Jul 22 16:05:38 p8h61 dhcpcd[674]: enp4s0: IAID 00:d0:73:4d
Jul 22 16:05:38 p8h61 dhcpcd[674]: enp4s0: IA type 25 IAID 00:00:00:00
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: soliciting a DHCP lease
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: soliciting an IPv6 router
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: offered 87.92.92.12 from 62.241.198.74
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: probing address 87.92.92.12/18
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: soliciting a DHCPv6 lease
Jul 22 16:05:39 p8h61 dhcpcd[674]: br0: activating for delegation
Jul 22 16:05:39 p8h61 dhcpcd[674]: br0: IAID cd:8a:cb:71
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: Router Advertisement from fe80::bb
Jul 22 16:05:39 p8h61 dhcpcd[674]: enp4s0: adding default route via fe80::bb
Jul 22 16:05:41 p8h61 dhcpcd[674]: enp4s0: ADV 2001:14ba:a0f9:2500::/56 from fe80::bb
Jul 22 16:05:41 p8h61 dhcpcd[674]: enp4s0: REPLY6 received from fe80::bb
Jul 22 16:05:41 p8h61 dhcpcd[674]: enp4s0: renew in 1800, rebind in 2880, expire in 3600 seconds
Jul 22 16:05:41 p8h61 dhcpcd[674]: lo: adding reject route to 2001:14ba:a0f9:2500::/56
Jul 22 16:05:41 p8h61 dhcpcd[674]: enp4s0: delegated prefix 2001:14ba:a0f9:2500::/56
Jul 22 16:05:41 p8h61 dhcpcd[674]: br0: adding address 2001:14ba:a0f9:2505::1/64
Jul 22 16:05:41 p8h61 dhcpcd[674]: br0: adding route to 2001:14ba:a0f9:2505::/64
Jul 22 16:05:45 p8h61 dhcpcd[674]: enp4s0: leased 87.92.92.12 for 3600 seconds
Jul 22 16:05:45 p8h61 dhcpcd[674]: enp4s0: adding route to 87.92.64.0/18
Jul 22 16:05:45 p8h61 dhcpcd[674]: enp4s0: adding default route via 87.92.64.1

What 'ip a' gives me:

enp4s0 starts with a DHCPv4 valid_lft 3600sec preferred_lft 2880sec, while br0 starts with both IPv6 values at 3600sec, which flat out drops IPv6 entirely at every 3600sec.