Add support for config directories
    
    Option '-C|--configdir' has been added, and defaults to /etc/gssproxy.  File
    "gssproxy.conf" and all files of the form "##-foo.conf" will be read from that
    directory.
    
    Ticket: https://fedorahosted.org/gss-proxy/ticket/122
    Signed-off-by: Robbie Harwood <rharwood@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>