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.