Port to GLnxTmpfile

There's lots of mechanically replacing `OtTmpFile` with `GLnxTmpfile`;
the biggest changes are in the commit path.  Symlink commits are now
very clearly separated from regular files.  Symlinks are `OtCleanupUnlinkat`,
and regular files are `GLnxTmpfile`.

The commit codepath separates those as `_ostree_repo_commit_path_final()` and
`_ostree_repo_commit_tmpf_final()`. A nice aspect of all of this is that they
both *consume* the temporary on success. This avoids an extra spurious
`unlink()` call.

One of the biggest bits of code motion is in `commit_loose_regfile_object()`,
which no longer needs to care about symlinks. For the most parth though it's
just removing conditionals.

Update submodule: libglnx

Closes: #958
Approved by: jlebon

src/libostree/ostree-repo-static-delta-processing.c

@@ -56,7 +56,7 @@ typedef struct {
   GError        **async_error;
 
   OstreeObjectType output_objtype;
-  OtTmpfile        tmpf;
+  GLnxTmpfile      tmpf;
   guint64          content_size;
   GOutputStream   *content_out;
   GChecksum       *content_checksum;
@@ -281,7 +281,7 @@ _ostree_static_delta_part_execute (OstreeRepo      *repo,
 
   ret = TRUE;
  out:
-  ot_tmpfile_clear (&state->tmpf);
+  glnx_tmpfile_clear (&state->tmpf);
   g_clear_object (&state->content_out);
   g_clear_pointer (&state->content_checksum, g_checksum_free);
   return ret;