Wireguard support for Mullvad and Windscribe (#565)

- `internal/wireguard` client package with unit tests
- Implementation works with kernel space or user space if unavailable
- `WIREGUARD_PRIVATE_KEY`
- `WIREGUARD_ADDRESS`
- `WIREGUARD_PRESHARED_KEY`
- `WIREGUARD_PORT`
- `internal/netlink` package used by `internal/wireguard`

Dockerfile

@@ -68,6 +68,7 @@ LABEL \
     org.opencontainers.image.description="VPN swiss-knife like client to tunnel to multiple VPN servers using OpenVPN, IPtables, DNS over TLS, Shadowsocks, an HTTP proxy and Alpine Linux"
 ENV VPNSP=pia \
     VERSION_INFORMATION=on \
+    VPN_TYPE=openvpn \
     PROTOCOL=udp \
     OPENVPN_VERSION=2.5 \
     OPENVPN_VERBOSITY=1 \
@@ -77,6 +78,11 @@ ENV VPNSP=pia \
     OPENVPN_IPV6=off \
     OPENVPN_CUSTOM_CONFIG= \
     OPENVPN_INTERFACE=tun0 \
+    WIREGUARD_PRIVATE_KEY= \
+    WIREGUARD_PRESHARED_KEY= \
+    WIREGUARD_ADDRESS= \
+    WIREGUARD_PORT= \
+    WIREGUARD_INTERFACE=wg0 \
     TZ= \
     PUID= \
     PGID= \