[kernel] Remove status code from thread exit

The status code from thread exit had too many issues, (eg, how does it
relate to process exit code? what happens when different threads exit
with different exit codes?) and not enough value, so I'm getting rid of
it.

src/user/drv.uart/main.cpp

@@ -83,7 +83,7 @@ channel_pump_loop()
 void
 pump_proc()
 {
-    j6_thread_exit(channel_pump_loop());
+    j6_process_exit(channel_pump_loop());
 }
 
 int