Also, add two more features "tx-tcp-segmentation" and
"tx-tcp6-segmentation". There are two reasons for that:

 - systemd-networkd supports setting these two features,
   so lets support them too (apparently they are important
   enough for networkd).

 - these two features are already implicitly covered by "tso".
   Like for the "ethtool" program, "tso" is an alias for several
   actual features. By adding two features that are already
   also covered by an alias (which sets multiple kernel names
   at once), we showcase how aliases for the same feature can
   coexist. In particular, note how setting
   "tso on tx-tcp6-segmentation off" will behave as one would
   expect: all 4 tso features covered by the alias are enabled,
   except that particular one.