[runner] Terminate the job's whole session, not just its shell

[un-def]

Previously, a job that did not stop on the interrupt was left to WaitDelay, which SIGKILLs cmd.Process -- the wrapper shell -- and nothing else. Everything the job had started went on running, reparented to PID 1, and was cleaned up only when the container was destroyed.

The interrupt cannot cover that gap on its own. The line discipline delivers it to the terminal's foreground process group, and job control puts a cmd & job in a process group of its own, where neither the interrupt nor the SIGHUP the kernel sends to the foreground group on the session leader's exit will reach it.

Now the job's session is signalled directly. A session is the right scope because a child inherits its parent's session id, and only setsid() changes it.

When the job is asked to stop, termination is staged: SIGHUP once hupDelay has passed, SIGKILL once killDelay has. SIGHUP is what a terminal hangup delivers and what an idle shell exits on, which also clears the shell dash leaves behind -- interrupted, it returns to reading the terminal rather than exiting. SIGCONT goes with it, since a stopped process would not act on the SIGHUP until something resumes it.

The stages run from a goroutine so they proceed while cmd.Wait is still blocked, and execJob joins it afterwards, because the processes a job leaves behind can outlive the shell that started them. Each stage is skipped when the session is already empty, so a job that stops on the interrupt is not delayed. Zombies are not occupants: they have exited and only await reaping.

When the job instead reaches its own end, its leftovers -- a sidecar started with &, say -- are sent the same hangup, but nothing is waited for. They are about to lose the terminal and then the container without being told either way, so they are told; the job succeeded, though, so there is nothing left to enforce and no reason to delay reporting it.

Exceeding the log quota now cancels the command's context instead of killing the shell outright, so it escalates the same way rather than through a second, blunter path.