First commit towards adding MacVtap link support
Small cleanup and refactoring.

Added validateOptions() function, cleaned up Link creation functions.
Also renamed MacVlanDev option to just Dev - its more concise and
can be used without much confusion in MacVtap interface.
Added macvtap tests. Renamed docknet_linux to tenus_linux.
Cleaned up VlanLink code a little bit

The cleanup was inspired by the clenaup in MacVlan/Vtap link.
Small other modifications added.
Replacing hardcoded default MacVlan mode with a constant.
Merge pull request #8 from milosgajdos83/macvtap