Unbreak guest access to samba shares

A change was made in samba 3.6.13 that made "force user" not work the
way it did before. The way to make it work now is

  map to guest = Bad User

instead of the default "map to guest = Never".

Relevant reading material:
  https://bugzilla.samba.org/show_bug.cgi?id=9746
  https://bbs.archlinux.org/viewtopic.php?id=160161