Support podman exec in krun

The krun backend does not support exec for "obvious" reasons (there is no existing mechanism for launching executables into the microVM). However, such a mechanism could be built. `muvm` is already doing something similar (cc @slp).

My motivation was using krun to run Forgejo runners and get a full GitHub Actions-style VM capable of running rootful podman/docker/everything else inside a rootless unprivileged container without running into corner cases. I thought I was being all clever and manually testing the concept worked perfectly (except for #2080):

```
[forgejo-runner@runner ~]$ podman --runtime krun run podman sh -c 'id; podman run --mount type=bind,source=/dev/null,target=/dev/mqueue alpine id'
uid=0(root) gid=0(root) groups=0(root)
Resolved "alpine" as an alias (/etc/containers/registries.conf.d/000-shortnames.conf)
Trying to pull docker.io/library/alpine:latest...
Getting image source signatures
Copying blob sha256:6a0ac1617861a677b045b7ff88545213ec31c0ff08763195a70a4a5adda577bb
Copying config sha256:3cb067eab609612d81b4d82ff8ad71d73482bb3059a87b642d7e14f0ed659cde
Writing manifest to image destination
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
```

But alas, when I actually plugged it into forgejo-runner:

Has this been considered before? libkrun already has an init binary and it could (optionally) fork off a server to handle `exec` commands, communicating via a vsocket or vport pair.

Aside: rootful-in-rootless also hits #2080, so that might be worth doing a release soon. This is the incantation required for non-krun rootful in rootless:

```
podman run --device /dev/fuse --security-opt unmask=ALL podman sh -c 'id; podman run --mount type=bind,source=/dev/null,target=/dev/mqueue alpine id'
uid=0(root) gid=0(root) groups=0(root)
[...]
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
```