Calling norns(ctl)?_error() to query the status of a running task will
release its descriptor in the server if the task already completed
(NORNS_EFINISHED or NORNS_EFINISHEDWERROR). Since norns(ctl)?_wait()
internally queries the task status to check whether a task has
completed, we cache the retrieved task_status into the iotask_t user
descriptor so that it can be retrieved by a subsequent
norns(ctl)?_error() call.

Transfer plugins now receive a new 'context' object that provides access
to relevant data structures from the daemon. This way, they can query
information about the current configuration and status without directly
involving the daemon object.

Closes #24