fsl wrapper: mv semantics

mv dir1 dir2 # dir2 NOT exists
```
      => dir2
```
mv dir1 dir2 # dir2 exists
```
      => dir2/dir1
```
mv f1 f2 # f2 not exists, otherwise error
```
      => f2
```
mv f1 dir1 # dir1 exists
```
      => dir1/f1
```
mv f1 dir1/f2 # dir1 may or may not exist, error if f2 exists
```
      => dir1/f2
```

mv f1 dir1 ../f2 dir2/f3 dir3 # dir3 exists (error otherwise)

      => dir3/dir1 
         dir3/f2 
         dir3/dir2/f3