fix handling of downloads of files > 2GB

git-svn-id: https://svn.r-project.org/R/trunk@66814 00db46b3-68df-0310-9c12-caf00c1e9a41
be more explicit about nano*.c copyrights

git-svn-id: https://svn.r-project.org/R/trunk@66816 00db46b3-68df-0310-9c12-caf00c1e9a41
remove Unix-only change

git-svn-id: https://svn.r-project.org/R/trunk@66818 00db46b3-68df-0310-9c12-caf00c1e9a41