sd-ndisc: better validate RA packets Verify the hoplimit and that the received packet is large enough for the RA header. See <http://tools.ietf.org/html/rfc4861#section-6.1.2>.

sd-ndisc: always send the link-layer address We never send packets without first knowing the link-local L3 address, so we should always include the L2 address in RS packets.

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