Linux SDL GUI + bug fixes + printf->slog

- sdl-gui/sdl.c is a tiny, basically working SDL2-based GUI that can compile and run on linux.
  It still needs PRAM integration and a config file.

- Fixed a dumb bug in shoebill_initialize that was causing sporadic crashes

- Replaced a million printf() calls with slog(), to make the SDL client run more quietly on the CLI

- Added more corruption checking to alloc_pool