wstream: write completion callback

Now modules using the wstream can find out what's happening to their writes.
job: add in-pipe notification and closing support

- One can now manually close the in-pipe, without having to tear down the
  job.
- One can be notified of write success/failure.
SQUASH/shell.c: support process input

This is needed to support the second argument of the `f_system()` call.
SQUASH/test: add stdin passing test