dhcp: fix dhcpv6 client and dhcpv6 prefix delegation

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 <[email protected]>
Change-Id: I68ec1c52cc1f4a8aa256185820748b845e92f7c1
(cherry picked from commit 20b962d3e494513ab28854936cc8bbc32891686b)