os: cap reads and writes to 2GB on Darwin and FreeBSD
os: cap reads and writes to 2GB on Darwin and FreeBSD

Fixes Issue 7812

LGTM=josharian, iant
R=rsc, iant, adg, ruiu, minux.ma, josharian
CC=golang-codereviews
https://codereview.appspot.com/89900044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang