exec: forward signals to the exec session's process group

[sahilnyk]

Fixes: #19486

Killing podman exec left the command it started running inside the container, because the signal was never passed on.

It now signals the process group of the exec'd command. The kernel already puts that command and its children in one group, so there is no process tree to walk.

podman exec --no-session had the same bug and is fixed too.

Tested on runc and crun, rootless and rootful. The new case in 032-sig-proxy.bats fails without this change.

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Does this PR introduce a user-facing change?

Fixed a bug where killing `podman exec` would leave the command it started running inside the container.