Features:
+* systemd-nspawn should get the same SSH key support that vmspawn now has.
+
* insert the new pidfs inode number as a third field into PidRef, so that
PidRef are reasonably serializable without having to pass around fds.
* use udev rule networkd ownership property to take ownership of network
interfaces nspawn creates
+* mountfsd/nsresourced
+ - userdb: maybe allow callers to map one uid to their own uid
+ - bpflsm: allow writes if resulting UID on disk would be userns' owner UID
+ - make encrypted DDIs work (password…)
+ - add API for creating a new file system from scratch (together with some
+ dm-integrity/HMAC key). Should probably work using systemd-repart (access
+ via varlink).
+ - add api to make an existing file "trusted" via dm-integry/HMAC key
+ - port: portabled
+ - port: tmpfiles, sysusers and similar
+ - lets see if we can make runtime bind mounts into unpriv nspawn work
+
* add a kernel cmdline switch (and cred?) for marking a system to be
"headless", in which case we never open /dev/console for reading, only for
writing. This would then mean: systemd-firstboot would process creds but not