Whenever initializing new chroots, save the date in $WORK/workdir.cfg.
Add pmb.config.workdir.chroots_outdated() to check if it's time to zap
the chroots or not (since we don't update them automatically). Mark them
as outdated after two days.
This will be the first check in "pmbootstrap status" (future patches).
Related: #1829
Oliver Smith