Initial sketch of SIOCETHTOOL ioctl support

For now this only supports a single command (ETHTOOL_GWOL)

TODO:
 * Complete ioctl_ethtool_data_len for all commands
 * Add some more commands to /umockdev-ioctl-tree/ethtool test
 * Add integration test case that calls "ethtool" on a real-life ioctl record.

https://github.com/martinpitt/umockdev/issues/31