pkg/archive: Canonicalize stored paths

Currently pkg/archive stores nested windows files with
backslashes (e.g. `dir\`, `dir\file.txt`) and this causes
tar not being correctly extracted on Linux daemon.

This change assures we canonicalize all paths to unix
paths and add them to tar with that name independent of platform.

Fixes the following test cases for Windows CI:
- TestBuildAddFileWithWhitespace
- TestBuildCopyFileWithWhitespace
- TestBuildAddDirContentToRoot
- TestBuildAddDirContentToExistingDir
- TestBuildCopyDirContentToRoot
- TestBuildCopyDirContentToExistDir
- TestBuildDockerignore
- TestBuildEnvUsage
- TestBuildEnvUsage2

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

archive/archive_unix.go

@@ -9,6 +9,13 @@ import (
 	"github.com/docker/docker/vendor/src/code.google.com/p/go/src/pkg/archive/tar"
 )
 
+// canonicalTarNameForPath returns platform-specific filepath
+// to canonical posix-style path for tar archival. p is relative
+// path.
+func canonicalTarNameForPath(p string) (string, error) {
+	return p, nil // already unix-style
+}
+
 func setHeaderForSpecialDevice(hdr *tar.Header, ta *tarAppender, name string, stat interface{}) (nlink uint32, inode uint64, err error) {
 	s, ok := stat.(*syscall.Stat_t)