Adds possibility to change another user password

[github at samba.org]

New comment by hortimech on Samba Github repository

https://github.com/samba-team/samba/pull/42#issuecomment-174479123
Comment:
As far as I am aware, the code in source4 is for Samba 4.x.x and samba-tool only works with Samba 4.x.x. I think you are also missing the fact that Samba 3.x.x is EOL.

I have no idea if your code is good code or otherwise, I just don't want something changing for a corner case if this may adversely affect others.