pam_get_authtok_verify: Avoid duplicate password verification

pam_get_authtok_verify: Avoid duplicate password verification

If password was already verified by previous modules in the stack
it does not need to be verified by pam_get_authtok_verify either.

* libpam/pam_get_authtok.c (pam_get_authtok_internal): Set the authtok_verified
  appropriately.
  (pam_get_authtok_verify): Do not prompt if authtok_verified is set and
  set it when the password is verified.
* libpam/pam_private.h: Add authtok_verified to the pam handle struct.
* libpam/pam_start.c (pam_start): Initialize authtok_verified.