We must ensure we only use supported backends, but as this contains
PostgreSQL, MySQL and SQLite, I see no blocker there.

This reverts merge request !261

Close #287

This change modifies the TOML parsing behaviour to accept something like either…

  [ldap.groupz]
  type = "posixGroup"
  search = { base = "ou=Groups,dc=example,dc=com" }

…or…

  [ldap.groupz]
  type = "posixGroup"
  search = [ { base = "ou=Groups,dc=example,dc=com" }, { base = "ou=Groups,dc=example2,dc=com" } ]

…or…

  [ldap.groupz]
  type = "posixGroup"

  [[ldap.groups.search]]
  base = "ou=Groups,dc=example,dc=com"

  [[ldap.groups.search]]
  base = "ou=Groups,dc=example2,dc=com"