Restructure source tree.

Specifically:
  * Elminate the kernel/ directory.
  * Move lib-like files to src/lib.
  * Move non-lib-like files to src/kernel.
  * Cause Makefile to use src/lib as an include directory,
    so we can do #include <string.h>, for example.
Add vga_write_newline to vga.h.

Eventually, it will probably be made non-public.  But, as long as
we're calling it from src/kernel, it shoul be in vga.h.
Don't compare unsigned and signed ints!