this is needed to support file loading: we want a device struct
so we can enumerate probes from the file, but there is no plugin
since the data come in from a device.
We have SIGROK_OK for functions calls where no errors occured. All
error code names start with SIGROK_ERR and are globally unique,
negative values.
The value SIGROK_ERR is a generic/unspecified error code, all others,
such as SIGROK_ERR_MALLOC, refer to a specific error condition.
This commit renames the old SIGROK_NOK etc.