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/helpers.go

@@ -0,0 +1,103 @@
+package util
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path"
+	"runtime"
+	"strings"
+)
+
+// ContainsString checks if "v" is in the array "arr"
+func ContainsString(arr []string, v string) bool {
+	for _, i := range arr {
+		if i == v {
+			return true
+		}
+	}
+	return false
+}
+
+// fileExists returns TRUE if file exists
+func FileExists(fn string) bool {
+	_, err := os.Stat(fn)
+	if err != nil {
+		return false
+	}
+	return true
+}
+
+// runCommand runs shell command
+func RunCommand(command string, arguments ...string) (int, string, error) {
+	cmd := exec.Command(command, arguments...)
+	out, err := cmd.Output()
+	if err != nil {
+		return 1, "", fmt.Errorf("exec.Command(%s) failed: %v: %s", command, err, string(out))
+	}
+
+	return cmd.ProcessState.ExitCode(), string(out), nil
+}
+
+func FuncName() string {
+	pc := make([]uintptr, 10) // at least 1 entry needed
+	runtime.Callers(2, pc)
+	f := runtime.FuncForPC(pc[0])
+	return path.Base(f.Name())
+}
+
+// SplitNext - split string by a byte and return the first chunk
+// Skip empty chunks
+// Whitespace is trimmed
+func SplitNext(str *string, splitBy byte) string {
+	i := strings.IndexByte(*str, splitBy)
+	s := ""
+	if i != -1 {
+		s = (*str)[0:i]
+		*str = (*str)[i+1:]
+		k := 0
+		ch := rune(0)
+		for k, ch = range *str {
+			if byte(ch) != splitBy {
+				break
+			}
+		}
+		*str = (*str)[k:]
+	} else {
+		s = *str
+		*str = ""
+	}
+	return strings.TrimSpace(s)
+}
+
+// MinInt - return the minimum value
+func MinInt(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// IsOpenWrt checks if OS is OpenWRT
+func IsOpenWrt() bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+
+	body, err := ioutil.ReadFile("/etc/os-release")
+	if err != nil {
+		return false
+	}
+
+	return strings.Contains(string(body), "OpenWrt")
+}
+
+// IsFreeBSD checks if OS is FreeBSD
+func IsFreeBSD() bool {
+	if runtime.GOOS == "freebsd" {
+		return true
+	}
+
+	return false
+}