This is a change to unreleased new behaviour to handle the clients.readonly.dir
data as part of the database swap handling.
Added a disk space safety check, so the refresh_P4ROOT_from_offline_db will
safely abort early if there's not enough disk space to safely complete the
rsync of the readonly.clients.dir folder to the offline_db prior to the
database swap orchestrated by refresh_P4ROOT_from_offline_db.sh.
Renamed the copy function for improved clarity, and added a new rsync_with_preflight
function to improve modularity.