Add more flexible _remote_change() API , expose via 'ostree remote'

For Anaconda, I needed OSTREE_REPO_REMOTE_CHANGE_ADD_IF_NOT_EXISTS,
with the GFile *sysroot argument to avoid ugly hacks.  We want to
write the content provided via "ostreesetup" as a remote to the target
chroot only in the case where it isn't provided as part of the tree
content itself.

This is also potentially useful in idempotent systems management tools
like Ansible.

https://bugzilla.gnome.org/show_bug.cgi?id=741577