syscalls can return negative i/o counts. fix bugs in ReadAt and WriteAt not to include negative counts in return values.
syscalls can return negative i/o counts. fix bugs in ReadAt and WriteAt not to include
negative counts in return values.

R=rsc
CC=golang-dev
http://codereview.appspot.com/170044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go