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-174477252
Comment:
I kept the first one to allow to easily understand what the second does (the version 4 of samba source code is very difficult to read and understand). Anyway, I removed the first commit, it only remains the source 4 commit.