Only show size for regular files

This avoids showing a meaningless '0 B' (or misleading arbitrary other
size) for folders.  Other special files also lack a meaningful size, so
don't show them either.

Symlinks are a bit of a tricky case here: we currently show the size
of the link (which is the length of the link target in bytes).  We could
show the size of the target, but this is a bit odd.  Let's show nothing.

Fixes #531

src/files-card-body.jsx

@@ -418,7 +418,7 @@ const Row = React.memo(function Item({ file, isSelected }) {
               className="item-size"
               dataLabel="size"
             >
-                {cockpit.format_bytes(file.size)}
+                {file.type === 'reg' && cockpit.format_bytes(file.size)}
             </Td>
             <Td
               className="item-date"