allow for intermediate stage in stopping acquisition

allow for intermediate stage in stopping acquisition

In the case of USB drivers, a driver's dev_acquisition_stop() cannot
simply remove its fd sources from the session and close its devices:
a USB transfer might still be underway, and it needs to be finished
(and its memory freed) properly.

An sr_dev_inst->status value is added: SR_ST_STOPPING, which should
be set when the driver's dev_acquisition_stop() is called, and acts
as a marker for the USB event handler to wind up its operations.

In order for dev_acquisition_stop() to be able to set the sdi status,
however, it needs to be unconstified.