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`

internal/configuration/wireguard.go

@@ -0,0 +1,73 @@
+package configuration
+
+import (
+	"fmt"
+	"net"
+	"strings"
+
+	"github.com/qdm12/golibs/params"
+)
+
+// Wireguard contains settings to configure the Wireguard client.
+type Wireguard struct {
+	PrivateKey   string     `json:"privatekey"`
+	PreSharedKey string     `json:"presharedkey"`
+	Address      *net.IPNet `json:"address"`
+	Interface    string     `json:"interface"`
+}
+
+func (settings *Wireguard) String() string {
+	return strings.Join(settings.lines(), "\n")
+}
+
+func (settings *Wireguard) lines() (lines []string) {
+	lines = append(lines, lastIndent+"Wireguard:")
+
+	lines = append(lines, indent+lastIndent+"Network interface: "+settings.Interface)
+
+	if settings.PrivateKey != "" {
+		lines = append(lines, indent+lastIndent+"Private key is set")
+	}
+
+	if settings.PreSharedKey != "" {
+		lines = append(lines, indent+lastIndent+"Pre-shared key is set")
+	}
+
+	if settings.Address != nil {
+		lines = append(lines, indent+lastIndent+"Address: "+settings.Address.String())
+	}
+
+	return lines
+}
+
+func (settings *Wireguard) read(r reader) (err error) {
+	settings.PrivateKey, err = r.env.Get("WIREGUARD_PRIVATE_KEY",
+		params.CaseSensitiveValue(), params.Unset(), params.Compulsory())
+	if err != nil {
+		return fmt.Errorf("environment variable WIREGUARD_PRIVATE_KEY: %w", err)
+	}
+
+	settings.PreSharedKey, err = r.env.Get("WIREGUARD_PRESHARED_KEY",
+		params.CaseSensitiveValue(), params.Unset())
+	if err != nil {
+		return fmt.Errorf("environment variable WIREGUARD_PRESHARED_KEY: %w", err)
+	}
+
+	addressString, err := r.env.Get("WIREGUARD_ADDRESS", params.Compulsory())
+	if err != nil {
+		return fmt.Errorf("environment variable WIREGUARD_ADDRESS: %w", err)
+	}
+	ip, ipNet, err := net.ParseCIDR(addressString)
+	if err != nil {
+		return fmt.Errorf("environment variable WIREGUARD_ADDRESS: %w", err)
+	}
+	ipNet.IP = ip
+	settings.Address = ipNet
+
+	settings.Interface, err = r.env.Get("WIREGUARD_INTERFACE", params.Default("wg0"))
+	if err != nil {
+		return fmt.Errorf("environment variable WIREGUARD_INTERFACE: %w", err)
+	}
+
+	return nil
+}