job: Add defer flag and setter method

This is has the same effect as the RStream 'defer' flag, but also works for the
job's exit event.

src/nvim/eval.c

@@ -10476,6 +10476,7 @@ static void f_job_start(typval_T *argvars, typval_T *rettv)
             on_job_stdout,
             on_job_stderr,
             on_job_exit,
+            true,
             &rettv->vval.v_number);
 
   if (rettv->vval.v_number <= 0) {