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/command.go

@@ -0,0 +1,28 @@
+package openvpn
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+)
+
+func (c *configurator) Start() (stdout io.ReadCloser, err error) {
+	c.logger.Info("%s: starting openvpn", logPrefix)
+	stdout, _, _, err = c.commander.Start("openvpn", "--config", string(constants.OpenVPNConf))
+	return stdout, err
+}
+
+func (c *configurator) Version() (string, error) {
+	output, err := c.commander.Run("openvpn", "--version")
+	if err != nil && err.Error() != "exit status 1" {
+		return "", err
+	}
+	firstLine := strings.Split(output, "\n")[0]
+	words := strings.Fields(firstLine)
+	if len(words) < 2 {
+		return "", fmt.Errorf("openvpn --version: first line is too short: %q", firstLine)
+	}
+	return words[1], nil
+}