Fix exclusion logic at thread creation and allow main thread to exit.
* libnpth.vers (npth_create, npth_exit): New. * npth.def (npth_create, npth_exit): Likewise. * npth.h (npth_create): Change from macro to declaration. (npth_exit): New declaration. * npth.c (startup_s): New struct. (thread_startup, npth_create, npth_exit): New functions.