sd-icmp6-nd: Add Router Solicitation and Advertisement support

Provide functions to bind the ICMPv6 socket to the approriate interface
and set multicast sending and receiving according to RFC 3493, section
5.2. and RFC 3542, sections 3. and 3.3. Filter out all ICMPv6 messages
except Router Advertisements for the socket in question according to
RFC 3542, section 3.2.

Send Router Solicitations to the all routers multicast group as
described in RFC 4861, section 6. and act on the received Router
Advertisments according to section 6.3.7.

Implement a similar API for ICMPv6 handling as is done for DHCPv4 and
DHCPv6.