config: Add code to source flag filters
    
    2 New configuration options are made available:
    - filter_flags
    - enforce_flags
    
    Any GSS Flags listed in the filter_flags option is forcibly filtered
    out before a gss_init_sec_context() call is invoked.
    Any GSS Flags listed in the enforce_flags option is forcibly added
    to the list of flags requested by a gss_init_sec_context() call is
    invoked.
    
    Flags can be either literals or numeric and must be preceded by the
    sign + (to add to the list) or - (to remove from the list).
    
    Resolves: https://fedorahosted.org/gss-proxy/ticket/109
    
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>