lib/pull: Add OSTREE_REPO_PULL_FLAGS_BAREUSERONLY_FILES

lib/pull: Add OSTREE_REPO_PULL_FLAGS_BAREUSERONLY_FILES

This is an option which is intended mostly for flatpak;
see: https://github.com/flatpak/flatpak/issues/845

We're adding an option for pulling into *all*
repo modes that has an effect similar to the `bare-user-only`
change from https://github.com/ostreedev/ostree/pull/909

This way one can pull content into e.g. a root-owned `bare` repository and
ensure that there aren't any setuid or world-writable files.

Closes: #926
Approved by: alexlarsson