rtnl: add support for receiving route messages
rtnl: clean up/add asserts
rtnl: match - only match on one type at a time
rtnl: simplify link_new()

Drop most of the arguments and instead introduce link_set_{flags,type}.
rtnl: simplify route_new()

Drop most of the arguments and instead introduce set_dst_prefixlen().
rtnl: support interleaved reading and writing, and rewind
rtnl: replace message_append by typesafe versions
network: use SETLINK to bring up interfaces