Cleanups of pam_pwhistory code. Make opasswd entry parsing more robust.

Cleanups of pam_pwhistory code. Make opasswd entry parsing more robust.

        * modules/pam_pwhistory/opasswd.c (check_old_password): Do not
        needlessly call strdupa().
        (save_old_password): Avoid memleaks in error paths. Avoid memleak of
        buf. Make the opasswd entry parsing more robust.
        * modules/pam_pwhistory/pam_pwhistory.8.xml: Document the
        special meaning of remember=0.