NetworkChange: Support Mac OS X (bcx#12038)

Refactor System.Net.NetworkingInformation in order
to support multiple "providers"; previously only
Linux was ever supported for this API.

This patch provides basic support for Mac OS X
via the SystemConfiguration framework's
SCNetworkReachability API.

bxc#12038
Merge pull request #627 from abock/master

Support Mac OS X for System.Net.NetworkInformation.NetworkChange