The current implementation in protocol-usb matches the new
documentation. The rationale for the change is to provide quick feedback
to the host that the device is not ready to handle requests yet. This is
not a big change because this part of the board API is not exposed to
the applet API. It is exclusively used by the scheduler. We might want
to do similar (and bigger) changes to other parts of the API (like UART
or USB serial).

Part of #577 because the runner host without USB still needs to support
the platform protocol.

Fixes #600

Also improve the behavior on exit. In particular, fix the runner exiting
properly when the web client is closed.

This is part of #577.

Part of #577.

Part of #577. This is going to be used by `AppletExitStatus` too.

Co-authored-by: Zhou Fang <33002388+zhou-w-fang@users.noreply.github.com>
Co-authored-by: Julien Cretin <cretin@google.com>

This will be useful for #577 which will introduce a file for the applet.

Fixes #577

Also revert #618 since this is a better fix (`i686-unknown-linux-gnu`
should only be used to compile a device, not a host, so it should not
depend on `rusb`).

No need to compile one. We only use cargo check in testing (not cargo
build), so we won't attempt to link the applet. The fact that we don't
link is also important because the existing native applet could have
been for another target triple.

This is part of #278 and similar to #630.

This is needed for #278 since ideally want a single release of the host
runner. And we still want this release to provide support for different
transports. In particular because USB requires sudo and doesn't work in
GitHub Codespaces.