pam_unix: Use pam_get_authtok() instead of direct pam_prompt() calls.

pam_unix: Use pam_get_authtok() instead of direct pam_prompt() calls.

We have to drop support for not_set_pass option which is not much useful
anyway. Instead we get proper support for authtok_type option.

* modules/pam_unix/pam_unix.8.xml: Removed not_set_pass option, added authtok_ty
pe
option.
* modules/pam_unix/pam_unix_auth.c (pam_sm_authenticate): Replace _unix_read_pas
sword()
call with equivalent pam_get_authtok() call.
* modules/pam_unix/pam_unix_passwd.c (pam_sm_chauthtok): Likewise and also drop
support for not_set_pass.
* modules/pam_unix/support.c (_unix_read_password): Remove.
* modules/pam_unix/support.h: Remove UNIX_NOT_SET_PASS add UNIX_AUTHTOK_TYPE.