os: Plan 9: avoid doing zero-length writes.

Plan 9 I/O preserves message boundaries, while Go
library code is written for UNIX-like operating
systems which do not. Avoid doing zero-length
writes in package os.

R=rsc, rminnich, ality, rminnich, r
CC=golang-dev
https://codereview.appspot.com/7406046

Committer: Ron Minnich <rminnich@gmail.com>