Fix #3852, make enable_rdp with other languages

* Tried to retrieve ```%ERRORLEVEL%``` after the ```net user /add``` command, unfortunately it's 0 when the command works or fails. So doesn't look like an option here.

* Decided to add a second check after the original one. And it's to check if the user is listed with a ```net user``` command. In order to work, a check has been also added before the ```net user /add``` command. If the user already exists, hasn't sense to try to add it (it will fail).