* pam_fshadow: skip-password option

Based on the proposal of Mirsad Goran Todorovac, the new option
skip-password instructs pam_fshadow to check whether the user
being authenticated is present in the passwd and/or shadow files,
without verifying his password.  This way pam_fshadow can be used as
an auxiliary module in the stack, actual authentication being
performed by one of the modules before it.