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>

src/os/dir_unix.go

@@ -15,6 +15,28 @@ const (
 	blockSize = 4096
 )
 
+func (f *File) readdir(n int) (fi []FileInfo, err error) {
+	dirname := f.name
+	if dirname == "" {
+		dirname = "."
+	}
+	names, err := f.Readdirnames(n)
+	fi = make([]FileInfo, 0, len(names))
+	for _, filename := range names {
+		fip, lerr := lstat(dirname + "/" + filename)
+		if IsNotExist(lerr) {
+			// File disappeared between readdir + stat.
+			// Just treat it as if it didn't exist.
+			continue
+		}
+		if lerr != nil {
+			return fi, lerr
+		}
+		fi = append(fi, fip)
+	}
+	return fi, err
+}
+
 func (f *File) readdirnames(n int) (names []string, err error) {
 	// If this file has no dirinfo, create one.
 	if f.dirinfo == nil {