In this patch I am reworking the logic around detecting virtual port support and requiring the libnl dependency.

- It requires --with-macvtap and displays an error in case of --without-macvtap --with-virtualport.
- It tests for availability of certain data in include files and displays an error in case the include file is not at the correct level and --with-virtualport was chosen
- displays 'checking' messages for macvtap and virtualport support and results
- libnl support is required when macvtap is found or requested; if libnl is not there, please supply without-macvtap