nsswitch: Add try_authok option to pam_winbind

[github at samba.org]

New comment by sathieu on Samba Github repository

https://github.com/samba-team/samba/pull/168#issuecomment-395322196
Comment:
I don't understand why the `echo_off_list` (`[password, newpassword, newpassword]`) is not passed to pam_winbind. Any clues?

Also, as those tests change the password, how should we handle password policy (i.e min password age)?