Adds possibility to change another user password

[github at samba.org]

New comment by aaptel on Samba Github repository

https://github.com/samba-team/samba/pull/42#issuecomment-172826888
Comment:
Additionally, since the logon name has a maximum length your patch could be simplified.

From https://technet.microsoft.com/it-it/library/bb726984%28en-us%29.aspx:
>Logon names can be up to 104 characters. However, it isn't practical to use logon names that are longer than 64 characters.