[ipv6] Treat sin6_scope_id consistently

sin6_scope_id is never exposed outside of the local system, and so
should be a native-endian quantity.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Automatically choose source for link-local and multicast destinations

When transmitting to a link-local or multicast destination address,
use the network device's link-local address as the source address if
no explicit source address has been specified.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pxe] Always retrieve cached DHCPACK and apply to relevant network device

When chainloading, always retrieve the cached DHCPACK packet from the
underlying PXE stack, and apply it as the original contents of the
"net<X>.dhcp" settings block.  This allows cached DHCP settings to be
used for any chainloaded iPXE binary (not just undionly.kkpxe).

This change eliminates the undocumented "use-cached" setting.  Issuing
the "dhcp" command will now always result in a fresh DHCP request.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge commit '10d19bd2ac302cd5237e7b1ca200760594a3c8c2'