Windows has a public dns.Flush used in router_windows.go.
However that won't work for platforms like Linux, where
we need a different flush mechanism for resolved versus
other implementations.
We're instead adding a FlushCaches method to the dns Manager,
which can be made to work on all platforms as needed.
Fixes https://github.com/tailscale/tailscale/issues/2132
Signed-off-by: Denton Gentry <dgentry@tailscale.com>
Denton Gentry
David Anderson