Linux: minclass in pam_pwquality.so

The pam_pwquality module can be plugged into the password stack of a given service to provide some plug-in strength-checking for passwords. The code was originally based on pam_cracklib module and the module is backwards compatible with its options. - minclass: The minimum number of required classes of characters for the new password. The default number is zero. The four classes are digits, upper and lower letters and other characters. The difference to the credit check is that a specific class if of characters is not required. Instead N out of four of the classes are required.