IANAL, but if you are not imposing a EULA, you shouldn't need any kind of license. End-user licenses restrict what can be done with the copy of the software that is owned. Licenses like the GPL restrict what can be done when redistributing the software, but impose nothing on the end-users. If you are not wanting to permit your end-users to redistribute, simple copyright is enough to protect your rights without the need for an additional license. If the software is not being redistributed and you aren't requiring a EULA, then the end-users are free to modify the software as they see fit (or do anything with it, except redistribute) under existing copyright law. So it seems copyright law as-is protects you from redistribution and permits your users the ability to modify the software, without the need of any license.