core: OOM really shouldn't be considered a deserialization parse failure
update TODO
manager: log deserialization errors only at LOG_DEBUG level

During upgrades and when transitioning between different systemd
versions in initrd and on the host we have to expect that some
serialization fields are unknown or parse incorrectly. This shouldn't
really be considered an error, hence downgrade the log messages about
it to debug. This way we can still trace it, but it doesn't confuse
users.

This kinda reverts 46849c3f.
systemctl: show unit file preset state in "systemctl status" output"
busctl: fix 'command line' style output of properties
bus-proxy: don't log bus policy every single time we run
rc-local: drop SysVStartPriority= field which is now obsolete
update TODO