"sshd -i" breaks due to socket activation

On Jammy and earlier, simply running "sshd -i" worked.

Now, it fails silently, and running it with "-d" gives me:

Missing privilege separation directory: /run/sshd

This directory is normally created with "RuntimeDirectory=sshd" as defined in /lib/systemd/system/ssh.service. In Jammy, this directory got created by the ssh service starting at boot, so "sshd -i" worked.

Now, with socket activation, it no longer does that, so "sshd -i" fails unless someone has actually connected on TCP port 22 (which they often won't have, since that's the point of "sshd -i").

systemd will then remove /run/sshd when the ssh service is stopped. I think maybe this won't interfere with an existing "sshd -i", but it's not really clean. Further, the privilege separation directory doesn't appear to be configurable - at least I couldn't find any mention in sshd_config(5).

The workaround is to "mkdir -p /run/sshd && sshd -i" instead.

Given that "sshd -i"'s use of /run/sshd isn't really related to the systemd service, maybe we should move the creation of that directory into tmpfiles.d instead?