Pull request: all: mv IsOpenWrt

Updates #2829.

Squashed commit of the following:

commit a284a26ba5df101c78e6f866d1bdfa540d205666
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 8 20:34:20 2021 +0300

    aghos: fix darwin

commit 9dbd42d75ebb048c83dbd06a1f09d950b3d12181
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 8 20:11:56 2021 +0300

    all: mv IsOpenWrt

internal/aghos/os_windows.go

@@ -40,3 +40,7 @@ func haveAdminRights() (bool, error) {
 func sendProcessSignal(pid int, sig syscall.Signal) error {
 	return fmt.Errorf("not supported on Windows")
 }
+
+func isOpenWrt() (ok bool) {
+	return false
+}