Features:
+* in the service manager, pick up ERRNO= + BUSERROR= + VARLINKERROR= error
+ identifiers, and store them along with the exit status of a server and report
+ via "systemctl status".
+
* enumerate virtiofs devices during boot-up in a generator, and synthesize
mounts for rootfs, /usr/, /home/, /srv/ and some others from it, depending on
- the "tag".
+ the "tag". (waits for: https://gitlab.com/virtio-fs/virtiofsd/-/issues/128)
* automatically mount one virtiofs during early boot phase to /run/host/,
similar to how we do that for nspawn, based on some clear tag.