Merge changes Ib327e8a3,I195dd388,I4875ac7f,I8479a8c2,Ia6fcde63, ...

* changes:
  Don't complain when deleting non-existent tethering rules.
  Don't call NetlinkCallbacks with nullptr on NLMSG_DONE.
  Use netlink code to flush rules.
  Add a generic netlink dump function and use it in SockDiag.
  Move the netlink command code to a new NetlinkCommands file.
  Put most of netd into the android::net namespace.