portability fixes / cleanup

include/my_sys.h:
  declaration should certainly match definition!
mysys/safemalloc.c:
  Make the various print-functions happy:
  sf_malloc_(cur|max)_memory are size_t now, might as well use %u instead
  of %d. Ideally, we'd use %zu, but we can't rely on having that, so we'll
  use %lu instead. Likewise, we could cast to unsigned for our poor man's
  %p -- pointers are never negative, and neither is %x --, but since it
  was fixed to %p with seemingly no ill effects in 6.0 anyway, we'll back
  port that instead.