apps/system/cu:  Add a some configurable defaults (only to reduce my keystrokes in repetitive testing)

Add a minimalist implemention  of the 'cu' terminal program (part of Taylor UUCP for ages). Using it, you can simply open a serial port and interact with it. Using '~.' you can leave the terminal program and drop back to nsh.

This might  come in handy for other people that have e.g. GSM modems, GPS receivers or other devices with text based serial communications attached to their Nuttx systems.

Traveler:  Add logic to limit the frame rate.  This is kind of a silly feature -- why would you ever want to limit the frame rate?  Well, you need to that on the simulated platform to make bandwidth for other things to run like the simulated timer