os: merge common Unix/Windows methods

Several method implementations were identical in file_unix.go and
file_windows.go. Merge them into file_posix.go.

Change-Id: I8bcfad468829530f81f52fe426b3a8c042e7bbd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/224138


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>