Adds possibility to change another user password

[github at samba.org]

New comment by GuillaumeGomez on Samba Github repository

https://github.com/samba-team/samba/pull/42#issuecomment-174477513
Comment:
@hortimech: Hum... Is it really the same thing? I took a look and didn't find any "obvious" way to do the thing I was looking for (and as far as I know, it wasn't possible in version 3, and that's the original reason of this PR).