udhcp6: fix problems found running against dnsmasq

Patch is based on work by tiggerswelt.net. They say:
"
But when we tried to use dnsmasq on server-side, udhcpc6 was unable to
forward the acquired address to its setup-script although the
IPv6-Address had been assigned by the server as we could see via
tcpdump. We traced this issue down to a problem on how udhcpc6 parses
DHCPv6-Options: When moving to next option, a pointer-address is
increased and a length buffer is decreased by the length of the option.
The problem is that it is done in this order:

  option += 4 + option[3];
  len_m4 -= 4 + option[3];

But this has to be switched as the length is decreased by the length of
the *next* option, not the current one. This affected both - internal
checks if a required option is present and the function to expose
options to the environment of the setup-script.
There was also a bug parsing D6_OPT_STATUS_CODE Options, that made
dnsmasq not work as udhcpc6 thought it is receiving a non-positive
status-code (because it did not parse the status-code as required in RFC
3315).
In addition we introduced basic support for RFC 3646 (OPTION_DNS_SERVERS
and OPTION_DOMAIN_LIST) and RFC 4704 (OPTION_CLIENT_FQDN).
"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

networking/udhcp/d6_common.h

@@ -81,9 +81,14 @@ struct d6_option {
 #define D6_OPT_RECONF_MSG    19
 #define D6_OPT_RECONF_ACCEPT 20
 
+#define D6_OPT_DNS_SERVERS   23
+#define D6_OPT_DOMAIN_LIST   24
+
 #define D6_OPT_IA_PD         25
 #define D6_OPT_IAPREFIX      26
 
+#define D6_OPT_CLIENT_FQDN   39
+
 /*** Other shared functions ***/
 
 struct client6_data_t {