Pull request: 2231 autoupdate

Merge in DNS/adguard-home from 2231-autoupdate to master

Updates #2231.

Squashed commit of the following:

commit 4ee9148ee7a38f2759898302a2109aa982fb4ee9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Nov 30 19:08:14 2020 +0300

    sysutil: provide os-independent interface

commit 778097c5fdeb1dec94f4cfc6443d08f92d9db0ba
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Nov 30 16:40:33 2020 +0300

    all: add sysutil package

internal/sysutil/os_unix.go

@@ -0,0 +1,32 @@
+//+build aix darwin dragonfly netbsd openbsd solaris
+
+package sysutil
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+func canBindPrivilegedPorts() (can bool, err error) {
+	return HaveAdminRights()
+}
+
+func setRlimit(val uint) {
+	var rlim syscall.Rlimit
+	rlim.Max = uint64(val)
+	rlim.Cur = uint64(val)
+	err := syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rlim)
+	if err != nil {
+		log.Error("Setrlimit() failed: %v", err)
+	}
+}
+
+func haveAdminRights() (bool, error) {
+	return os.Getuid() == 0, nil
+}
+
+func sendProcessSignal(pid int, sig syscall.Signal) error {
+	return syscall.Kill(pid, sig)
+}