pam_cracklib: Add maxclassrepeat, gecoscheck checks and remove unused difignore.

pam_cracklib: Add maxclassrepeat, gecoscheck checks and remove unused difignore.

modules/pam_cracklib/pam_cracklib.c (_pam_parse): Recognize the maxclassrepeat, gecoscheck options. Ignore difignore option.
(simple): Add the check for the same class repetition.
(usercheck): Refactor into wordcheck().
(gecoscheck): New test for words from the GECOS field.
(password_check): Call the gecoscheck().
(pam_sm_chauthtok): Drop the diff_ignore from options struct.
modules/pam_cracklib/pam_cracklib.8.xml: Document the maxclassrepeat and gecoscheck checks, update the documentation of the difok test.