job-control: mitigate process-kill race

children_kill_cb() is racey. One obvious problem is that
process_close_handles() is *queued* by on_process_exit(), so when
children_kill_cb() is invoked, the dead process might still be in the
`loop->children` list.  If the OS already reclaimed the dead PID, Nvim
may try to SIGKILL it.

Avoid that by checking `proc->status`.

Vim doesn't have this problem because it doesn't attempt to kill
processes that ignored SIGTERM after a timeout.

closes #8269

runtime/doc/eval.txt

@@ -5033,10 +5033,9 @@ jobstart({cmd}[, {opts}])				*jobstart()*
 		  width    : (pty only) Width of the terminal screen
 		  height   : (pty only) Height of the terminal screen
 		  TERM     : (pty only) $TERM environment variable
-		  detach   : (non-pty only) Detach the job process from the
-			     nvim process. The process will not get killed
-			     when nvim exits. If the process dies before
-			     nvim exits, "on_exit" will still be invoked.
+		  detach   : (non-pty only) Detach the job process: it will
+			     not be killed when Nvim exits. If the process
+			     exits before Nvim, "on_exit" will be invoked.
 
 		{opts} is passed as |self| dictionary to the callback; the
 		caller may set other keys to pass application-specific data.