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/openvpn/openvpn.go

@@ -0,0 +1,35 @@
+package openvpn
+
+import (
+	"io"
+	"os"
+
+	"github.com/qdm12/golibs/command"
+	"github.com/qdm12/golibs/files"
+	"github.com/qdm12/golibs/logging"
+)
+
+const logPrefix = "openvpn configurator"
+
+type Configurator interface {
+	Version() (string, error)
+	WriteAuthFile(user, password string, uid, gid int) error
+	CheckTUN() error
+	Start() (stdout io.ReadCloser, err error)
+}
+
+type configurator struct {
+	fileManager files.FileManager
+	logger      logging.Logger
+	commander   command.Commander
+	openFile    func(name string, flag int, perm os.FileMode) (*os.File, error)
+}
+
+func NewConfigurator(logger logging.Logger, fileManager files.FileManager) Configurator {
+	return &configurator{
+		fileManager: fileManager,
+		logger:      logger,
+		commander:   command.NewCommander(),
+		openFile:    os.OpenFile,
+	}
+}