all: Support syncing ownership (fixes #1329) (#8434)

This adds support for syncing ownership on Unixes and on Windows. The
scanner always picks up ownership information, but it is not applied
unless the new folder option "Sync Ownership" is set.

Ownership data is stored in a new FileInfo field called "platform data". This
is intended to hold further platform-specific data in the future
(specifically, extended attributes), which is why the whole design is a
bit overkill for just ownership.

lib/fs/filesystem.go

@@ -14,6 +14,8 @@ import (
 	"path/filepath"
 	"strings"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/protocol"
 )
 
 type filesystemWrapperType int32
@@ -30,7 +32,7 @@ const (
 // The Filesystem interface abstracts access to the file system.
 type Filesystem interface {
 	Chmod(name string, mode FileMode) error
-	Lchown(name string, uid, gid int) error
+	Lchown(name string, uid, gid string) error // uid/gid as strings; numeric on POSIX, SID on Windows, like in os/user package
 	Chtimes(name string, atime time.Time, mtime time.Time) error
 	Create(name string) (File, error)
 	CreateSymlink(target, name string) error
@@ -60,6 +62,7 @@ type Filesystem interface {
 	URI() string
 	Options() []Option
 	SameFile(fi1, fi2 FileInfo) bool
+	PlatformData(name string) (protocol.PlatformData, error)
 
 	// Used for unwrapping things
 	underlying() (Filesystem, bool)