Reduce warnings.

Pset 4 for 2019.

Pset 3 for 2019.

Support clang 5.0-style sanitizers.

Add p-nastyalloc; you will add a nasty system call in Part F.

Better to have a separate program for this.

Prepare for journal.

- Allocate a 64-block journal in chickadeefs.img.
- Correctly set `superblock::njournal`.
- Correct replay.
- Replay becomes more verbose.
- Add --help to chickadeefsck/mkchickadeefs.

Better interface to ahcistate::read/write.

sh: Ensure different lines are processed separately.

Use GCC assembly to generate the `pause` instruction.

Clang complains about the builtin because we disable SSE, etc.

Add p-allocexit program.