Implement support for IP tunnel flags. Currently only some IPv6 tunnel
flags are supported. Example:

 # nmcli connection add type ip-tunnel mode ip6ip6 \
   ip-tunnel.flags ip6-ign-encap-limit,ip6-use-orig-tclass \
   ifname abc ip-tunnel.parent ens8 ipv4.method disabled \
   ipv6.method manual ipv6.address ::8888 remote ::42

 # ip -d l
  61: abc@ens8: <NOARP,UP,LOWER_UP> mtu 1460 qdisc noqueue ...
    link/tunnel6 :: brd ::42 promiscuity 0
    ip6tnl ip6ip6 remote ::42 local :: dev ens8 encaplimit none
    hoplimit 0 tclass inherit ...

https://bugzilla.gnome.org/show_bug.cgi?id=791846