Start unification of libsigrok return codes.

Start unification of libsigrok return codes.

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.