Rewrite of the entrypoint in Golang (#71)

- General improvements
    - Parallel download of only needed files at start
    - Prettier console output with all streams merged (openvpn, unbound, shadowsocks etc.)
    - Simplified Docker final image
    - Faster bootup
- DNS over TLS
    - Finer grain blocking at DNS level: malicious, ads and surveillance
    - Choose your DNS over TLS providers
    - Ability to use multiple DNS over TLS providers for DNS split horizon
    - Environment variables for DNS logging
    - DNS block lists needed are downloaded and built automatically at start, in parallel
- PIA
    - A random region is selected if the REGION parameter is left empty (thanks @rorph for your PR)
    - Routing and iptables adjusted so it can work as a Kubernetes pod sidecar (thanks @rorph for your PR)

internal/firewall/firewall.go

@@ -0,0 +1,42 @@
+package firewall
+
+import (
+	"net"
+
+	"github.com/qdm12/golibs/command"
+	"github.com/qdm12/golibs/files"
+	"github.com/qdm12/golibs/logging"
+	"github.com/qdm12/private-internet-access-docker/internal/models"
+)
+
+const logPrefix = "firewall configurator"
+
+// Configurator allows to change firewall rules and modify network routes
+type Configurator interface {
+	Version() (string, error)
+	AcceptAll() error
+	Clear() error
+	BlockAll() error
+	CreateGeneralRules() error
+	CreateVPNRules(dev models.VPNDevice, serverIPs []net.IP, defaultInterface string,
+		port uint16, protocol models.NetworkProtocol) error
+	CreateLocalSubnetsRules(subnet net.IPNet, extraSubnets []net.IPNet, defaultInterface string) error
+	AddRoutesVia(subnets []net.IPNet, defaultGateway net.IP, defaultInterface string) error
+	GetDefaultRoute() (defaultInterface string, defaultGateway net.IP, defaultSubnet net.IPNet, err error)
+	AllowInputTrafficOnPort(device models.VPNDevice, port uint16) error
+}
+
+type configurator struct {
+	commander   command.Commander
+	logger      logging.Logger
+	fileManager files.FileManager
+}
+
+// NewConfigurator creates a new Configurator instance
+func NewConfigurator(logger logging.Logger, fileManager files.FileManager) Configurator {
+	return &configurator{
+		commander:   command.NewCommander(),
+		logger:      logger,
+		fileManager: fileManager,
+	}
+}