Pause key walk mode - may remove this once testing completes.
Basic threaded control (threading works, logic almost done):

- Compile for pthreads
- break out the difference between schedule_item and io_item
- Simple single flow nervous_system::control method
- nervous sytem now handles the keypresses for the walking
- Added reloop_pipe to break out of select to add things to select lists
First threaded nervous system

- cleaned up the state transitions for arms (STATIONARY, MOVING_FORWARD,
       MOVING_BACKWARD)
- arm can be declared in use for intent
- threaded single flow nervous system stops for keys between operations
- scheduler has threaded capable entry points for adding items
- threaded_control_mgr is working and is mostly safe. Needs some cleanup.