os: consolidate files

os: consolidate files

Code movement only.

If someone finds function 'foo' in "foo_linux.go",
they will expect that the Window version of 'foo' exists in "foo_windows.go".

Current code doesn't follow this manner.

For example, 'sameFile' exists in "file_unix.go",
"stat_plan9.go" and "types_windows.go".

The CL address that problem by following rules:

* readdir family => dir.go, dir_$GOOS.go
* stat family => stat.go, stat_$GOOS.go
* path-functions => path_$GOOS.go
* sameFile => types.go, types_$GOOS.go
* process-functions => exec.go, exec_$GOOS.go
* hostname => sys.go, sys_$GOOS.go

Change-Id: Ic3c64663ce0b2a364d7a414351cd3c772e70187b
Reviewed-on: https://go-review.googlesource.com/27035
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>