fix st command help hangs

Before running the command function global the main func would start the
print and error queues.  Inside the command function the the option
parser would see the the help option, print the help text, and raise
SystemExit, which wasn't getting caught.

bin/st

@@ -1723,7 +1723,7 @@ Example:
         error_thread.abort = True
         while error_thread.isAlive():
             error_thread.join(0.01)
-    except Exception:
+    except (SystemExit, Exception):
         for thread in threading_enumerate():
             thread.abort = True
         raise