Pull request: all: restore netbsd compat

Updates #3147.

Squashed commit of the following:

commit 4e1bcfd1561f9b927755c5024521d5508ef94b1f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 17 13:17:51 2021 +0300

    all: restore netbsd compat

internal/aghos/os_bsd.go

@@ -0,0 +1,36 @@
+// +build darwin netbsd openbsd
+
+package aghos
+
+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)
+}
+
+func isOpenWrt() (ok bool) {
+	return false
+}