sd-dhcp6-client: bind to link-local address This ensures that several DHCPv6 clients can run on separate interfaces simultaneously.

sd-dhcp6-client: allow multiple clients on host We need to enable SO_REUSEADDR in order for several sockets to be allowed to bind to the same port (even on different links).

util-lib: split out fd-related operations into fd-util.[ch] There are more than enough to deserve their own .c file, hence move them over.

libsystemd-network: split icm6-util.[ch] out of dhcp6 code

tree-wide: we place the opening bracket on the same line as the function name Let's do this everywhere the same way.

sd-dhcp6-client: Add functions to bind to DHCPv6 UDP socket Add a function that creates a UDP socket bound to the given interface and optionally to an IPv6 address. Add another function that will send the DHCPv6 UDP packet to its destination. Using IPV6_PKTINFO in setsockopt to bind the IPv6 socket to an interface is documented in section 4. of RFC 3542, "Advanced Sockets Application Program Interface (API) for IPv6" Add a define for DHCPv6 Relay Agents and Servers multicast address as its not available elsewhere.

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.