platform: create netlink messages directly without libnl-route-3
Instead of using libnl-route-3 library to serialize netlink messages, construct the netlink messages ourselves. This has several advantages: - Creating the netlink message ourself is actually more straight forward then having an intermediate layer between NM and the kernel. Now it is immediately clear, how a platform request translates to a netlink/kernel request. You can look at the kernel sources how a certain netlink attribute behaves, and then it's immediately clear how to set that (and vice versa). - Older libnl versions might have bugs or missing features for which we needed to workaround (often by offering a reduced/broken/untested functionality). Now we can get rid or workaround like _nl_has_capability(), check_support_libnl_extended_ifa_flags(), HAVE_LIBNL_INET6_TOKEN. Another example is a libnl bug when setting vlan ingress map which isn't even yet fixed in libnl upstream. - We no longer need libnl-route-3 at all and can drop that runtime requirement, saving some 400k. Constructing the messages ourselves also gives better performance because we don't have to create the intermediate libnl object. - In the future we will add more link-type support which is easier to support by basing directly on the plain kernel/netlink API, instead of requiring also libnl3 to expose this functionality. E.g. adding macvtap support: we already parsed macvtap properties ourselves because of missing libnl support. To *add* macvtap support, we also would have to do it ourself (or extend libnl).
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Thomas Haller