optim: changed postfixed ++ into prefix++ for non trivial operators, replaced test on std::list::size() by std::list::empty() (Patch from Phenom, modified)

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7627 b45a01b8-16f6-495d-af2f-9b41ad6348cc