Improved message validation and server unicast option support

Hello Steven,

I have worked on two patches for the odhcp6c client.
The first patch (7ea97a4) has improved message validation support as described in RFC3315 paragraph 15; error handling when socket primitives are called and some minor bugfixes.
The second patch (348cbc2) implements support for the server unicast option as described in RFC3315 paragraphs 18.1 and 22.12; the new functionality has been tested against an ISC DHCPv6 server implementation.
I have send these changes as patches to the OpenWRT devel mailing list; but it this way of working via github seems less complex for me.
If you have any remarks regarding the changes; just let me know.

Bye,
Hans