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

bash/ostree

@@ -745,6 +745,7 @@ _ostree_checkout() {
 _ostree_checksum() {
     local boolean_options="
         $main_boolean_options
+        --ignore-xattrs
     "
 
     case "$cur" in