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

src/libostree/libostree-devel.sym

@@ -19,6 +19,8 @@
 
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2017.13 {
+global:
+  ostree_checksum_file_at;
 } LIBOSTREE_2017.12;
 
 /* Stub section for the stable release *after* this development one; don't
@@ -27,6 +29,6 @@ LIBOSTREE_2017.13 {
  * with whatever the next version with new symbols will be.
 LIBOSTREE_2017.$NEWVERSION {
 global:
-	someostree_symbol_deleteme;
+  someostree_symbol_deleteme;
 } LIBOSTREE_2017.$LASTSTABLE;
 */