Support `FDSTORE=1` (sd_pid_notify_with_fds) to restart a container without losing active TCP connections

I would like to have a container use
```
sd_pid_notify_with_fds(0, 0, "FDSTORE=1\nFDNAME=foobar", &fd, 1);
```
(see [`man sd_notify`](https://www.freedesktop.org/software/systemd/man/sd_notify.html)) to store an active TCP socket. It would then be possible to restart a container that has an active TCP connection without the container losing the connection. (Containers generally don't use `sd_pid_notify_with_fds()` so it would only work for containers with support for it).

Scenario: `/usr/bin/testserver` in the container image _IMG:1_ supports _socket activation_. A client on the internet connects. The testserver is started and calls [`accept()`](https://man7.org/linux/man-pages/man2/accept.2.html) on the leaked-in socket. A new container image _IMG:2_ is released. The sysadmin wants to upgrade to _IMG:2_ without having to disconnect the active TCP connection.

The same scenario described in more detail:

1. `sudo useradd test1`
2. `sudo machinectl shell test1@`
3. `podman image tag IMG:1 tmptag:latest`
4. `podman create --rm --name test --network none tmptag:latest /usr/bin/testserver`
5. `mkdir -p ~/.config/systemd/user`
6. `podman generate systemd --name --new test > ~/.config/systemd/user/test.service`
7. create the file _~/.config/systemd/user/test.socket_ with the file contents
```
[Unit]
Description=test server
[Socket]
ListenStream=0.0.0.0:3000
[Install]
WantedBy=default.target
```
8. `systemctl --user start test.socket`
9. a client on the internet connects to TCP port 3000.
10. `/usr/bin/testserver` is started with the leaked-in socket (from _systemd socket activation_)
11. the testserver calls [`accept()`](https://man7.org/linux/man-pages/man2/accept.2.html) on the socket
12. A new container image image `IMG:2` is available.
`podman image tag IMG:2 tmptag:latest`
13. the sysadmin somehow informs the testserver that it needs to send the active TCP socket file descriptor to systemd with `FDSTORE=1`
14. testserver sends the active TCP socket file descriptor to systemd and gives it the name _foobar_
`sd_pid_notify_with_fds(0, 0, "FDSTORE=1\nFDNAME=foobar", &fd, 1);`
16. testserver terminates with an _unclean exit code_ (see [man systemd.service](https://www.freedesktop.org/software/systemd/man/systemd.service.html#Restart=)) so that systemd
will try to restart the service. (Alternatively the sysadmin could also run `systemctl --user restart test.service`).
17. `/usr/bin/testserver` is started again but this time it also inherits the file descriptor that was previously stored with `FDSTORE=1`.
18. testserver calls [`sd_listen_fds_with_names()`](https://www.freedesktop.org/software/systemd/man/sd_listen_fds_with_names.html#)

In the above example it was assumed that the `/usr/bin/testserver`is running stateless (except for having the active TCP connection). In a more realistic scenario `/usr/bin/testserver` would also need to store its internal application state before restarting. The normal file system could be used to save such a file, or even better `memfd_create(2)` could be used.
Copy-paste from [man sd_notify](https://www.freedesktop.org/software/systemd/man/sd_notify.html#FDSTORE=1):
_Application state can either be serialized to a file in /run/, or better, stored in a memory file descriptor. memfd_create(2)_ (See also [memfd_create(2)](http://man7.org/linux/man-pages/man2/memfd_create.2.html)).

In other words, it would be good if __conmon__ would also support sending a memfd with `FDSTORE=1`.

Maybe even more file descriptor types could be allowed to be sent? An idea: a new Podman command-line option could adjust what type of file descriptors are allowed to be sent.

Previous discussion:

* https://github.com/containers/podman/discussions/13570#discussioncomment-2449912

Extra note: I don't have any direct need for this feature right now. I just think it could be a useful feature.