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/pthread.c

@@ -132,3 +132,8 @@ threadexitsall(char *msg)
 	exits(msg);
 }
 
+void
+_threadpexit(void)
+{
+	pthread_exit(0);
+}