libnm, cli, ifcfg-rh: add NMSettingEthtool setting
Note that in NetworkManager API (D-Bus, libnm, and nmcli), the features are called "feature-xyz". The "feature-" prefix is used, because NMSettingEthtool possibly will gain support for options that are not only -K|--offload|--features, for example -C|--coalesce. The "xzy" suffix is either how ethtool utility calls the feature ("tso", "rx"). Or, if ethtool utility specifies no alias for that feature, it's the name from kernel's ETH_SS_FEATURES ("tx-tcp6-segmentation"). If possible, we prefer ethtool utility's naming. Also note, how the features "feature-sg", "feature-tso", and "feature-tx" actually refer to multiple underlying kernel features at once. This too follows what ethtool utility does. The functionality is not yet implemented server-side.
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