FIFO: Switch to C++ implementation.
C++: Rename all the files!
Remove linux_stuff.h, add missing bits to portapack.h.
Fix M4 command handler array indices.
Use ARRAY_SIZE for M4 command_handler array.
Fix #include for __RBIT().
snprintf and strnlen aren't available, so use non-N variants.
Replace array of pointers with std::vector.
UI: Add ui_widget_t parent to UI class(es).
UI: Change UI field init to C++11 style.
UI: Push ui_widget_t deep into code, use inheritance.