wgengine/router: move Tailscale's winipcfg additions into wgengine/router

Part of unforking our winipcfg-go and using upstream (#760), move our
additions into our repo. (We might upstream them later if upstream has
interest)

Originally these were:

@apenwarr: "Add ifc.SyncAddresses() and SyncRoutes()."
https://github.com/tailscale/winipcfg-go/commit/609dcf2df55fbb76effd430a51b9757676853390

@bradfitz: "winipcfg: make Interface.AddRoutes do as much as possible, return combined error"
https://github.com/tailscale/winipcfg-go/commit/e9f93d53f33a7d2cadbdfa822761b8251af4d4bd

@bradfitz: "prevent unnecessary Interface.SyncAddresses work; normalize IPNets in deltaNets"
https://github.com/tailscale/winipcfg-go/commit/decb9ee8e17028db14c6951413cdd34ab46efca4

cmd/tailscale/depaware.txt

@@ -5,7 +5,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         github.com/apenwarr/fixconsole                               from tailscale.com/cmd/tailscale
    W 💣 github.com/apenwarr/w32                                      from github.com/apenwarr/fixconsole
    L    github.com/coreos/go-iptables/iptables                       from tailscale.com/wgengine/router
-   L    github.com/go-multierror/multierror                          from tailscale.com/wgengine/router
+  LW    github.com/go-multierror/multierror                          from tailscale.com/wgengine/router
    W 💣 github.com/go-ole/go-ole                                     from github.com/go-ole/go-ole/oleutil+
    W 💣 github.com/go-ole/go-ole/oleutil                             from tailscale.com/wgengine/winnet
    L 💣 github.com/godbus/dbus/v5                                    from tailscale.com/wgengine/router/dns

cmd/tailscaled/depaware.txt

@@ -5,7 +5,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         github.com/apenwarr/fixconsole                               from tailscale.com/cmd/tailscaled
    W 💣 github.com/apenwarr/w32                                      from github.com/apenwarr/fixconsole
    L    github.com/coreos/go-iptables/iptables                       from tailscale.com/wgengine/router
-   L    github.com/go-multierror/multierror                          from tailscale.com/wgengine/router
+  LW    github.com/go-multierror/multierror                          from tailscale.com/wgengine/router
    W 💣 github.com/go-ole/go-ole                                     from github.com/go-ole/go-ole/oleutil+
    W 💣 github.com/go-ole/go-ole/oleutil                             from tailscale.com/wgengine/winnet
    L 💣 github.com/godbus/dbus/v5                                    from tailscale.com/wgengine/router/dns