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/errorfs.go

@@ -9,6 +9,8 @@ package fs
 import (
 	"context"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/protocol"
 )
 
 type errorFilesystem struct {
@@ -18,7 +20,7 @@ type errorFilesystem struct {
 }
 
 func (fs *errorFilesystem) Chmod(name string, mode FileMode) error { return fs.err }
-func (fs *errorFilesystem) Lchown(name string, uid, gid int) error { return fs.err }
+func (fs *errorFilesystem) Lchown(name, uid, gid string) error     { return fs.err }
 func (fs *errorFilesystem) Chtimes(name string, atime time.Time, mtime time.Time) error {
 	return fs.err
 }
@@ -52,6 +54,9 @@ func (fs *errorFilesystem) SameFile(fi1, fi2 FileInfo) bool { return false }
 func (fs *errorFilesystem) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, <-chan error, error) {
 	return nil, nil, fs.err
 }
+func (fs *errorFilesystem) PlatformData(name string) (protocol.PlatformData, error) {
+	return protocol.PlatformData{}, fs.err
+}
 
 func (fs *errorFilesystem) underlying() (Filesystem, bool) {
 	return nil, false