restore old plan 9 property that when the

last thread exits the main proc, the remaining
program ends up in the background and the
program appears to have exited.

src/libthread/threadimpl.h

@@ -122,3 +122,4 @@ extern int _threadspawn(int*, char*, char**);
 extern int _runthreadspawn(int*, char*, char**);
 extern void _threadsetupdaemonize(void);
 extern void _threaddodaemonize(char*);
+extern void _threadpexit(void);