net/vmnet: add vmnet dependency and customizable option

net/vmnet: add vmnet dependency and customizable option

vmnet.framework dependency is added with 'vmnet' option
to enable or disable it. Default value is 'auto'.

used vmnet features are available since macOS 11.0,
but new backend can be built and work properly with
subset of them on 10.15 too.

Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Vladislav Yaroshchuk <Vladislav.Yaroshchuk@jetbrains.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>