Fix #3852, make enable_rdp work 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).

Verification
--------------

- [x] Install Windows 7 (other language than English). I've used spanish.
- [x] Get a meterpreter session with administrator privileges
- [x] Try to run enable_rdp with "ENABLE false" and "USERNAME" and "PASSWORD" set. Use a username already existent in the system. It should fail:

```
msf post(enable_rdp) > run

[*] Setting user account for logon
[*] 	Adding User: test4 with Password: test1
[-] 	The user test4 already exists
[*] For cleanup execute Meterpreter resource file: /Users/jvazquez/.msf4/loot/20141212172832_default_172.16.158.133_host.windows.cle_120451.txt
[*] Post module execution completed
```
- [x] Try to run enable_rdp with "ENABLE false" and "USERNAME" and "PASSWORD" set. Use a non existent username. The module should work correctly:

```
msf post(enable_rdp) > run

[*] Setting user account for logon
[*] 	Adding User: test6 with Password: test1
[*] 	Adding User: test6 to local group 'Usuarios de escritorio remoto'
[*] 	Hiding user from Windows Login screen
[*] 	Adding User: test6 to local group 'Administradores'
[*] You can now login with the created user
[*] For cleanup execute Meterpreter resource file: /Users/jvazquez/.msf4/loot/20141212173943_default_172.16.158.133_host.windows.cle_334145.txt
[*] Post module execution completed
```