dhcp: fix dhcpv6 client and dhcpv6 prefix delegation

Keep trying even if the interface in question is not "admin-up,
link-up." In real life, it's normal for link autonegotiation to take a
good fraction of a second. The driver layer takes care of packets sent
to an interface which can't transmit at the moment.

Renew address leases at the preferred renewal time, not at the
expiration time.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I68ec1c52cc1f4a8aa256185820748b845e92f7c1