dhcpv6c: fix illegal DHCPV6_OPT_FQDN

dhcpv6c: fix illegal DHCPV6_OPT_FQDN

Some message types e.g. DHCPV6_MSG_INFO_REQ contained
DHCPV6_CLIENT_FQDN (39) which is illegal. See Client FQDN RFC4704:

https://www.rfc-editor.org/rfc/rfc4704#section-5

   A client MUST only include the Client FQDN option in SOLICIT,
   REQUEST, RENEW, or REBIND messages.

https://www.rfc-editor.org/rfc/rfc4704#section-6

   Servers MUST only include a Client FQDN option in ADVERTISE and REPLY
   messages...

Do not send it unless the message type is appropriate.

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/100
Signed-off-by: Álvaro Fernández Rojas <[email protected]>