local fs rename: if it fails with a cross device error try a copy

I don't want to add a new setting for this, at least until we get the
first complain for a slow rename :)

Fixes #440

vfs/sys_unix.go

@@ -0,0 +1,36 @@
+// +build !windows
+
+package vfs
+
+import (
+	"errors"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+var (
+	defaultUID, defaultGID int
+)
+
+func init() {
+	defaultUID = os.Getuid()
+	defaultGID = os.Getuid()
+	if defaultUID < 0 {
+		defaultUID = 65534
+	}
+	if defaultGID < 0 {
+		defaultGID = 65534
+	}
+}
+
+func (fi FileInfo) getFileInfoSys() interface{} {
+	return &syscall.Stat_t{
+		Uid: uint32(defaultUID),
+		Gid: uint32(defaultGID)}
+}
+
+func isCrossDeviceError(err error) bool {
+	return errors.Is(err, unix.EXDEV)
+}