DHCPv6 destination address check

As described in RFC3315 §15 any solicit, confirm, rebind or information request message is discarded if the destination address is unicast
Likewise any request (§18.2.1), renew (§18.2.3), release (§18.2.6) or decline (§18.2.7) message is discarded and the server replies with the status code use multicast.
Replace option sol_max_rt by inf_max_rt in reply response to information request
Merge pull request #18 from dedeckeh/bugfixes

Improved compliance with RFC3315
odhcpd: only enable packet socket if really needed
Merge remote-tracking branch 'upstream/master'