Add better default behaviors for concrete_fs
Add better default behaviors for concrete_fs

- Raise an error if you provide both fs and concrete_fs
- Try to start the state in the host cwd if you provide concrete_fs but
  not cwd
- Try to translate the host cwd into the chroot in the above case with a
  chroot specified

See angr/angr-doc#299

angr/angr,schieb/angr,angr/angr,angr/angr,schieb/angr,schieb/angr