stbt, stbt-batch: exec, instead of fork, the subcommand

...such as `stbt-run` or `stbt-batch.d/report`.

The change to `stbt-batch` fixes the tests
`test_stbt_batch_run_sigint_{once,twice}`.

It doesn't make any difference when running `stbt batch run`
interactively and pressing Control-C. That always worked, presumably
because the shell sends the sigint signal to all foreground processes in
the process group.

The change to `stbt.in` (which becomes the installed `stbt` script)
isn't strictly necessary, but I don't think it will hurt. Less
intermediate processes in the PID tree can only be a good thing.