Adds possibility to change another user password

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/42#issuecomment-163165561
Comment:
I understand it was the easiest to change, but we sadly can't accept such a patch to smbpasswd.

The easiest patch to write is sadly often not the best patch for the project as a whole.

I would suggest finding a way to patch 'samba-tool user password' to cover the auth/change user split you are after.  It is a combination of python and C, but perhaps you can patch py_net_change_password in python/samba/netcmd/py_net.c and python/samba/netcmd/user.py to have such an option.