migrate to golang.zx2c4.com/wireguard/wgctrl (#239)

* migrate to golang.zx2c4.com/wireguard/wgctrl

This commit introduces the usage of wgctrl.
It avoids the usage of exec calls of the wg command
and parsing the output of `wg show`.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* vendor wgctrl

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* apply suggestions from code review

Remove wireguard.Enpoint struct and use net.UDPAddr for the resolved
endpoint and addr string (dnsanme:port) if a DN was supplied.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/*: use wireguard.Enpoint

This commit introduces the wireguard.Enpoint struct.
It encapsulates a DN name with port and a net.UPDAddr.
The fields are private and only accessible over exported Methods
to avoid accidental modification.

Also iptables.GetProtocol is improved to avoid ipv4 rules being applied
by `ip6tables`.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/wireguard/conf_test.go: add tests for Endpoint

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* cmd/kg/main.go: validate port range

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* add suggestions from review

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/mesh/mesh.go: use Equal func

Implement an Equal func for Enpoint and use it instead of comparing
strings.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* cmd/kgctl/main.go: check port range

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* vendor

Signed-off-by: leonnicolas <leonloechner@gmx.de>

vendor/golang.zx2c4.com/wireguard/wgctrl/CONTRIBUTING.md

@@ -0,0 +1,23 @@
+Contributing
+============
+
+The `wgctrl` project makes use of the [GitHub Flow](https://guides.github.com/introduction/flow/)
+for contributions.
+
+If you'd like to contribute to the project, please
+[open an issue](https://github.com/WireGuard/wgctrl-go/issues/new) or find an
+[existing issue](https://github.com/WireGuard/wgctrl-go/issues) that you'd like
+to take on.  This ensures that efforts are not duplicated, and that a new feature
+aligns with the focus of the rest of the repository.
+
+Once your suggestion has been submitted and discussed, please be sure that your
+code meets the following criteria:
+
+- code is completely `gofmt`'d
+- new features or codepaths have appropriate test coverage
+- `go test ./...` passes
+- `go vet ./...` passes
+- `staticcheck ./...` passes
+- `golint ./...` returns no warnings, including documentation comment warnings
+
+Finally, submit a pull request for review!