Use existing PAX headers for time

This removes the MSWINDOWS.* time fields and uses PAX times directly. I
didn't previously realize that PAX times could be negative, which
eliminates the need to store Windows times in a different format.

This also uses the LIBARCHIVE.creationtime field for creation times to
match OS X behavior.

archive/tar/common.go

@@ -44,22 +44,23 @@ const (
 // A Header represents a single header in a tar archive.
 // Some fields may not be populated.
 type Header struct {
-	Name       string    // name of header file entry
-	Mode       int64     // permission and mode bits
-	Uid        int       // user id of owner
-	Gid        int       // group id of owner
-	Size       int64     // length in bytes
-	ModTime    time.Time // modified time
-	Typeflag   byte      // type of header entry
-	Linkname   string    // target name of link
-	Uname      string    // user name of owner
-	Gname      string    // group name of owner
-	Devmajor   int64     // major number of character or block device
-	Devminor   int64     // minor number of character or block device
-	AccessTime time.Time // access time
-	ChangeTime time.Time // status change time
-	Xattrs     map[string]string
-	Winheaders map[string]string
+	Name         string    // name of header file entry
+	Mode         int64     // permission and mode bits
+	Uid          int       // user id of owner
+	Gid          int       // group id of owner
+	Size         int64     // length in bytes
+	ModTime      time.Time // modified time
+	Typeflag     byte      // type of header entry
+	Linkname     string    // target name of link
+	Uname        string    // user name of owner
+	Gname        string    // group name of owner
+	Devmajor     int64     // major number of character or block device
+	Devminor     int64     // minor number of character or block device
+	AccessTime   time.Time // access time
+	ChangeTime   time.Time // status change time
+	CreationTime time.Time // creation time
+	Xattrs       map[string]string
+	Winheaders   map[string]string
 }
 
 // File name constants from the tar spec.
@@ -180,21 +181,22 @@ const (
 
 // Keywords for the PAX Extended Header
 const (
-	paxAtime    = "atime"
-	paxCharset  = "charset"
-	paxComment  = "comment"
-	paxCtime    = "ctime" // please note that ctime is not a valid pax header.
-	paxGid      = "gid"
-	paxGname    = "gname"
-	paxLinkpath = "linkpath"
-	paxMtime    = "mtime"
-	paxPath     = "path"
-	paxSize     = "size"
-	paxUid      = "uid"
-	paxUname    = "uname"
-	paxXattr    = "SCHILY.xattr."
-	paxWindows  = "MSWINDOWS."
-	paxNone     = ""
+	paxAtime        = "atime"
+	paxCharset      = "charset"
+	paxComment      = "comment"
+	paxCtime        = "ctime" // please note that ctime is not a valid pax header.
+	paxCreationTime = "LIBARCHIVE.creationtime"
+	paxGid          = "gid"
+	paxGname        = "gname"
+	paxLinkpath     = "linkpath"
+	paxMtime        = "mtime"
+	paxPath         = "path"
+	paxSize         = "size"
+	paxUid          = "uid"
+	paxUname        = "uname"
+	paxXattr        = "SCHILY.xattr."
+	paxWindows      = "MSWINDOWS."
+	paxNone         = ""
 )
 
 // FileInfoHeader creates a partially-populated Header from fi.