gfs2_edit: Add compression to savemeta and restoremeta
    
    This patch adds the ability to output gzip-compressed data with savemeta and
    makes it the default behaviour. It also adds a -z <0-9> option to allow the
    level of compression to be controlled, 0 meaning no compression and 9 being the
    default. restoremeta can now restore from a gzip-compressed or raw metadata
    file without any extra options.
    
    The file opening, closing and writing code from savemeta has been moved into
    savemeta{open,close,write} functions to abstract away compressed and
    non-compressed file output.
    
    Adds a dependency on zlib.
    
    rhbz#702296
    
    Signed-off-by: Andrew Price <anprice@redhat.com>
    Reviewed-by: Steven Whitehouse <swhiteho@redhat.com>