[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>