But this is all kind of moot if Peter is right that systemd will zap POSIX shmem along with SysV semaphores. I've been trying to reproduce the issue on a Fedora 25 installation, and so far I can't get it to zap anything, so I'm a bit at a loss how to prove things one way or the other.
Don't know precisely about Fedora 25, but I've had success in the past with:
ssh in as the user
start postgres under tmux/screen
logout
do another ssh login/logout cycle
After logon, you should see "/usr/lib/systemd/systemd --user" running for that
user. After logout out, said proc should exit. If either of those is not true,
either systemd is not setup to track sessions (probably via pam) or it thinks
you still have an active logon. Another way to check if systemd thinks the user