gfs2_edit: Use metadata description to print and assign fields
    
    Replace the large number of separate field assignment and printing
    functions in gfs2_edit with the new functions in libgfs2. This reduces
    the amount of code involved dramatically and fixes three coverity
    complaints related to the use of the 'checkassigns' macro.
    
    This relates to the form: gfs2_edit -p 1234 field foo [bar] /dev/baz
    
    The 'device' variable has been changed from char[NAME_MAX] to a pointer
    to an argv member, so that now we can check whether the optional [bar]
    argument is missing properly, i.e. no longer assume the device path will
    begin with '/'. It also means the device name is no longer limited to
    255 chars.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>