Implemented full TunClient class for Windows with TAP driver
support. For now, we use netsh (rather than TAP driver DHCP)
to set all tunnel adapter properties, as this appears to work
great on Windows 7.
IPv6 is fully supported.
Known isues:
* netsh doesn't have a command for adding DNS search domains, so
we don't support them yet.
* While we always try to remove routes and added properties from
TAP adapter instance when we close out the session, for robustness,
when we bring up TAP adapter, we should try to delete any stale
routes on interface left over from previous session.
* Right now we call netsh with system(). For security and
compatibility with Windows apps (not only console apps),
we should use CreateProcess instead.
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