lib/core: add ostree_checksum_file_at API

This is like `ostree_checksum_file` but fd-relative. This will be used
by https://github.com/ostreedev/ostree/pull/1258.

AFAICT, we actually didn't have any tests that check the `checksum` CLI.
Add a basic one here to test the old code as well as the new code.

Closes: #1263
Approved by: cgwalters

man/ostree-checksum.xml

@@ -61,6 +61,19 @@ Boston, MA 02111-1307, USA.
         </para>
     </refsect1>
 
+    <refsect1>
+        <title>Options</title>
+        <variablelist>
+            <varlistentry>
+                <term><option>--ignore-xattrs</option></term>
+
+                <listitem><para>
+                    Ignore extended attributes when checksumming.
+                </para></listitem>
+            </varlistentry>
+        </variablelist>
+    </refsect1>
+
     <refsect1>
         <title>Example</title>
         <para><command>$ ostree checksum file1</command></para>