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.go

@@ -45,3 +45,8 @@ func RunCommand(command string, arguments ...string) (int, string, error) {
 
 	return cmd.ProcessState.ExitCode(), string(out), nil
 }
+
+// IsOpenWrt returns true if host OS is OpenWrt.
+func IsOpenWrt() (ok bool) {
+	return isOpenWrt()
+}