[dhcpv6] Do not set sin6_scope_id on the unspecified client socket address

Setting sin6_scope_id to a non-zero value will cause the check against
the "empty socket address" in udp_demux() to fail, and incoming DHCPv6
responses on interfaces other than net0 will be rejected with a
spurious "No UDP connection listening on port 546" error.

The transmitting network device is specified via the destination
address, not the source address.  Fix by simply not setting
sin6_scope_id on the client socket address.

Reported-by: Anton D. Kachalov <mouse@yandex-team.ru>
Tested-by: Anton D. Kachalov <mouse@yandex-team.ru>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Do not set sin6_scope_id on source address

The transmitting network device is specified via the destination
address, not the source address.  There is no reason to set
sin6_scope_id on the source address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Include network device when transcribing multicast addresses

Destination multicast addresses require a sin6_scope_id, which should
therefore be transcribed to a network device name by ipv6_sock_ntoa().

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Avoid potentially copying from a NULL pointer in ipv6_tx()

If ipv6_tx() is called with a non-NULL network device, a NULL or
unspecified source address, and a destination address which does not
match any routing table entry, then it will attempt to copy the source
address from a NULL pointer.

I don't think that there is currently any code path which could
trigger this behaviour, but we should probably ensure that it can
never happen.

Signed-off-by: Michael Brown <mcb30@ipxe.org>