Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/util/syslog_windows.go

@@ -0,0 +1,39 @@
+package util
+
+import (
+	"log"
+	"strings"
+
+	"golang.org/x/sys/windows"
+	"golang.org/x/sys/windows/svc/eventlog"
+)
+
+type eventLogWriter struct {
+	el *eventlog.Log
+}
+
+// Write sends a log message to the Event Log.
+func (w *eventLogWriter) Write(b []byte) (int, error) {
+	return len(b), w.el.Info(1, string(b))
+}
+
+func ConfigureSyslog(serviceName string) error {
+	// Note that the eventlog src is the same as the service name
+	// Otherwise, we will get "the description for event id cannot be found" warning in every log record
+
+	// Continue if we receive "registry key already exists" or if we get
+	// ERROR_ACCESS_DENIED so that we can log without administrative permissions
+	// for pre-existing eventlog sources.
+	if err := eventlog.InstallAsEventCreate(serviceName, eventlog.Info|eventlog.Warning|eventlog.Error); err != nil {
+		if !strings.Contains(err.Error(), "registry key already exists") && err != windows.ERROR_ACCESS_DENIED {
+			return err
+		}
+	}
+	el, err := eventlog.Open(serviceName)
+	if err != nil {
+		return err
+	}
+
+	log.SetOutput(&eventLogWriter{el: el})
+	return nil
+}