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/dns.go

@@ -0,0 +1,70 @@
+package util
+
+import (
+	"net"
+	"strings"
+)
+
+// convert character to hex number
+func charToHex(n byte) int8 {
+	if n >= '0' && n <= '9' {
+		return int8(n) - '0'
+	} else if (n|0x20) >= 'a' && (n|0x20) <= 'f' {
+		return (int8(n) | 0x20) - 'a' + 10
+	}
+	return -1
+}
+
+// parse IPv6 reverse address
+func ipParseArpa6(s string) net.IP {
+	if len(s) != 63 {
+		return nil
+	}
+	ip6 := make(net.IP, 16)
+
+	for i := 0; i != 64; i += 4 {
+
+		// parse "0.1."
+		n := charToHex(s[i])
+		n2 := charToHex(s[i+2])
+		if s[i+1] != '.' || (i != 60 && s[i+3] != '.') ||
+			n < 0 || n2 < 0 {
+			return nil
+		}
+
+		ip6[16-i/4-1] = byte(n2<<4) | byte(n&0x0f)
+	}
+	return ip6
+}
+
+// ipReverse - reverse IP address: 1.0.0.127 -> 127.0.0.1
+func ipReverse(ip net.IP) net.IP {
+	n := len(ip)
+	r := make(net.IP, n)
+	for i := 0; i != n; i++ {
+		r[i] = ip[n-i-1]
+	}
+	return r
+}
+
+// DNSUnreverseAddr - convert reversed ARPA address to a normal IP address
+func DNSUnreverseAddr(s string) net.IP {
+	const arpaV4 = ".in-addr.arpa"
+	const arpaV6 = ".ip6.arpa"
+
+	if strings.HasSuffix(s, arpaV4) {
+		ip := strings.TrimSuffix(s, arpaV4)
+		ip4 := net.ParseIP(ip).To4()
+		if ip4 == nil {
+			return nil
+		}
+
+		return ipReverse(ip4)
+
+	} else if strings.HasSuffix(s, arpaV6) {
+		ip := strings.TrimSuffix(s, arpaV6)
+		return ipParseArpa6(ip)
+	}
+
+	return nil // unknown suffix
+}