core: Use at-relative functions for checking out tree copies too

For the cases where we can't hardlink, use at-relative walking of the
path where possible.  We still don't have lsetxattrat, so we also need
to deal with pathnames, but that is now only for symlinks.

Again, the advantages of this are a lot less malloc() of pathnames in
ostree, and much less time spent traversing paths inside the kernel.

https://bugzilla.gnome.org/show_bug.cgi?id=707733

src/libostree/ostree-core-private.h

@@ -83,6 +83,12 @@ _ostree_set_xattrs_fd (int            fd,
 gboolean _ostree_set_xattrs (GFile *f, GVariant *xattrs,
                              GCancellable *cancellable, GError **error);
 
+gboolean
+_ostree_make_temporary_symlink_at (int             tmp_dirfd,
+                                   const char     *target,
+                                   char          **out_name,
+                                   GCancellable   *cancellable,
+                                   GError        **error);
 
 /* XX + / + checksum-2 + . + extension, but let's just use 256 for a
  * bit of overkill.