Although extremely unlikely, there is a race present in solely checking the
$LISTEN_PID environment variable, due to PID recycling. Fix that by introducing
$LISTEN_PIDFDID, which contains the 64-bit ID of a pidfd for the child process
that is not subject to recycling.
This reverts commit 5e8ff010a1.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
ispell made some suggestions which I applied.
Addresses: https://github.com/systemd/systemd/pull/29209#pullrequestreview-1632623460
Also adds a brief paragraph about initrd transitions. (Plymouth really
should start using the fdstore for pinning DRM objects, and stop trying
to survive the initrd→host transition)
