Pull request: create aghnet package

Merge in DNS/adguard-home from mk-aghnet to master

Updates #2829.

Squashed commit of the following:

commit 519806c04b8d0517aacce9c31f2d06ab56631937
Merge: 92376c86 97361234
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 19:13:56 2021 +0300

    Merge branch 'master' into mk-aghnet

commit 92376c8665e529191aa482432f9d5e3e2e3afdc8
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 18:37:22 2021 +0300

    aghnet: fix linux

commit 7f36d19b0e650d4e4fc5cf9ea4b501a7f636abeb
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 18:08:30 2021 +0300

    aghnet: mv network utils from util

commit aa68c70c1146b8c32303c6e037953a41aa7d72f9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 17:30:06 2021 +0300

    aghnet: mv ipDetector here

commit b033657f5c5ee91f869c36508a5eb15976a174a0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 17:24:07 2021 +0300

    all: mk aghnet package, rename sysutil package

internal/aghos/os_windows.go

@@ -0,0 +1,42 @@
+// +build windows
+
+package aghos
+
+import (
+	"fmt"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+func canBindPrivilegedPorts() (can bool, err error) {
+	return HaveAdminRights()
+}
+
+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")
+}