[ipv6] Expose router address for DHCPv6 leased addressesHEADslash128master

The DHCPv6 protocol does not itself provide a router address or a
prefix length.  This information is instead obtained from the router
advertisements.

Our IPv6 minirouting table construction logic will first construct an
entry for each advertised prefix, and later update the entry to
include an address assigned within that prefix via stateful DHCPv6 (if
applicable).

This logic fails if the address assigned via stateful DHCPv6 does not
fall within any of the advertised prefixes (e.g. if the network is
configured to use DHCPv6-assigned /128 addresses with no advertised
on-link prefixes).  We will currently treat this situation as
equivalent to having a manually assigned address with no corresponding
router address or prefix length: the routing table entry will use the
default /64 prefix length and will not include the router address.

DHCPv6 is triggered only in response to a router advertisement with
the "Managed Address Configuration (M)" or "Other Configuration (O)"
flags set, and a router address is therefore available at the point
that we initiate DHCPv6.

Record the router address when initiating DHCPv6, and expose this
router address as part of the DHCPv6 settings block.  This allows the
routing table entry for any address assigned via stateful DHCPv6 to
correctly include the router address, even if the assigned address
does not fall within an advertised prefix.

Also provide a fixed /128 prefix length as part of the DHCPv6 settings
block.  When an address assigned via stateful DHCPv6 does not fall
within an advertised prefix, this will cause the routing table entry
to have a /128 prefix length as expected.  (When such an address does
fall within an advertised prefix, it will continue to use the
advertised prefix length.)

Originally-fixed-by: Guvenc Gulce <guevenc.guelce@sap.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

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