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/.cibuild.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+set -e
+set -x
+
+# !! This script is meant for use in CI build use only !!
+
+KERNEL=$(uname -s)
+
+# Use doas in place of sudo for OpenBSD.
+SUDO="sudo"
+if [ "${KERNEL}" == "OpenBSD" ]; then
+    SUDO="doas"
+fi
+
+if [ "${KERNEL}" == "Linux" ]; then
+    # Configure a WireGuard interface.
+    sudo ip link add wg0 type wireguard
+    sudo ip link set up wg0
+fi
+
+# Set up wireguard-go on all OSes.
+git clone git://git.zx2c4.com/wireguard-go
+cd wireguard-go
+
+if [ "${KERNEL}" == "Linux" ]; then
+    # Bypass Linux compilation restriction.
+    make
+else
+    # Build directly to avoid Makefile.
+    go build -o wireguard-go
+fi
+
+${SUDO} mv ./wireguard-go /usr/local/bin/wireguard-go
+cd ..
+${SUDO} rm -rf ./wireguard-go