Mobile prerefactor (#680)

Small code cleaning in the iface package. These changes necessary to 
get a clean code in case if we involve more platforms. The OS related 
functions has been distributed into separate files and it has been 
mixed with not OS related logic. The goal is to get a clear picture 
of the layer between WireGuard and business logic.

iface/address.go

@@ -0,0 +1,29 @@
+package iface
+
+import (
+	"fmt"
+	"net"
+)
+
+// WGAddress Wireguard parsed address
+type WGAddress struct {
+	IP      net.IP
+	Network *net.IPNet
+}
+
+// parseWGAddress parse a string ("1.2.3.4/24") address to WG Address
+func parseWGAddress(address string) (WGAddress, error) {
+	ip, network, err := net.ParseCIDR(address)
+	if err != nil {
+		return WGAddress{}, err
+	}
+	return WGAddress{
+		IP:      ip,
+		Network: network,
+	}, nil
+}
+
+func (addr WGAddress) String() string {
+	maskSize, _ := addr.Network.Mask.Size()
+	return fmt.Sprintf("%s/%d", addr.IP.String(), maskSize)
+}