ipa-getkeytab: fix compiler warnings
    
    Make read_ipa_config and filter_keys static to avoid
    "no previous prototype" warnings.
    
    Use correct datatype of return value for ber_scanf to
    correct different signedness comparision.
    
    Fixed while working on https://pagure.io/freeipa/issue/8478
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>