os: File declaration clean-up

[tortuoise]

The os.File type is re-declared across multiple files under src/os for os specific builds.

Can this type declaration be moved into src/os/file.go which currently contains the build-agnostic exported methods ? The declaration can then be removed from all the individual build specific file_xxxx.go ? Will this break anything else ?

If it doesn't break anything, this change will clean up the documentation as well and make it build agnostic because currently some of the type descriptions in the package documentation link to build specific source files, for eg type File links to src/os/file_unix.go

[gopherbot]

CL https://golang.org/cl/28511 mentions this issue.