Recursive calls to main_menu function

[brettski74]

There are numerous calls to main_menu() littered throughout the code. Since these calls all originate in functions that were themselves called from main_menu(), we have some infinite recursion going on. While it seems unlikely that the reflow plate would continue running long enough to exhaust the stack, it's unlikely that anything good will result if these recursive calls do eventually exhaust the stack. Code should be refactored to use the loop() function and operate as a state machine.