os: rename error_posix.go to error_unix.go

The tradition is to use _posix when the platform extends beyond unix variants. As windows has its own file, rename to the more usual _unix.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10320043

diff --git a/src/pkg/os/error_posix.go b/src/pkg/os/error_unix.go
similarity index 100%
rename from src/pkg/os/error_posix.go
rename to src/pkg/os/error_unix.go