Added Cloudius copyright statement to core/*.

poll.cc already had a BSD copyright statement, I believe this is a mistake
(I think Guy wrote this code from scratch), but not wanting to rush to a
conclusion I'm leaving both copyright statements and we should address this
issue later.

  sprintf(fmt, ...) - returns a std::stream
  fprintf(os, fmt, ...) - prints to a std::ostream&

Easier than the usualy method of constructing a temporary ostringstream
and formatting into that.