src/error.c: added a function to create an error like STk_error but without
raising it (it returns it). There is a lot of code duplication but I didn't know how to use it from STk_error because of the ... argument. all threads: renamed _specific to sys_ (sys_thread, sys_mutex, sys_condv) for code and data which are specific to a thread system. The _specific was confusing with TLS specific. all lurc thread: deal in the best way possible with lurc errors.
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