add job control

- in an interactive shell every pipeline runs in its own process group
  and gets the terminal while it runs in the foreground
- Ctrl+Z stops the foreground job, command & starts a job in the
  background; finished and stopped jobs are reported before the prompt
- new builtins jobs, fg, bg and wait (wait also works without job
  control, e.g. in scripts, where background jobs read from /dev/null)
- terminal modes are saved and restored when a job stops or continues
- exit and Ctrl+D warn once if there are stopped jobs
- processes are waited for with wait4 (WUNTRACED) instead of exec.Cmd.Wait
- pty based test for job control (Linux)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go.mod

@@ -7,4 +7,4 @@ require (
 	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510
 )
 
-require golang.org/x/sys v0.48.0 // indirect
+require golang.org/x/sys v0.48.0