Add support for unified samba packages
    
    Fedora 18 and later has moved unified samba and samba4 packages. Update
    Requires and BuildRequires in spec file to require correct versions.
    
    Also require libwbclient-devel which now provides libwbclient.h instead
    of samba4-devel package.
    
    https://fedorahosted.org/freeipa/ticket/3118