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/os_windows.go

@@ -0,0 +1,37 @@
+package util
+
+import (
+	"fmt"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// Set user-specified limit of how many fd's we can use
+func SetRlimit(val uint) {
+}
+
+func HaveAdminRights() (bool, error) {
+	var token windows.Token
+	h := windows.CurrentProcess()
+	err := windows.OpenProcessToken(h, windows.TOKEN_QUERY, &token)
+	if err != nil {
+		return false, err
+	}
+
+	info := make([]byte, 4)
+	var returnedLen uint32
+	err = windows.GetTokenInformation(token, windows.TokenElevation, &info[0], uint32(len(info)), &returnedLen)
+	token.Close()
+	if err != nil {
+		return false, err
+	}
+	if info[0] == 0 {
+		return false, nil
+	}
+	return true, nil
+}
+
+func SendProcessSignal(pid int, sig syscall.Signal) error {
+	return fmt.Errorf("not supported on Windows")
+}