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/env/env.go

@@ -0,0 +1,40 @@
+package env
+
+import (
+	"os"
+
+	"github.com/qdm12/golibs/logging"
+)
+
+type Env interface {
+	FatalOnError(err error)
+	PrintVersion(program string, commandFn func() (string, error))
+}
+
+type env struct {
+	logger logging.Logger
+	osExit func(n int)
+}
+
+func New(logger logging.Logger) Env {
+	return &env{
+		logger: logger,
+		osExit: os.Exit,
+	}
+}
+
+func (e *env) FatalOnError(err error) {
+	if err != nil {
+		e.logger.Error(err)
+		e.osExit(1)
+	}
+}
+
+func (e *env) PrintVersion(program string, commandFn func() (string, error)) {
+	version, err := commandFn()
+	if err != nil {
+		e.logger.Error(err)
+	} else {
+		e.logger.Info("%s version: %s", program, version)
+	}
+}