Authentication: scaffolding for multiple authentication backends

Instead of always putting users passwords in XML files, allow site
admins to configure a different authentication method to integrate
authentication with other systems.

This doesn't add any authentication backends yet, merely the
scaffolding. Notably:

- Password checking and loading/removing from storage has been
  decoupled. A new auth_check_pass function is used to check passwords.
  It does check against the configured storage first, but will handle
  the authentication backends as well. The XML storage merely signals
  that a user's password should be checked using an authentication
  backend.
- If unknown-to-bitlbee users identify using an authentication backend,
  they are automatically registered.
- If an authentication backend is used, that fact is stored in the XML
  file, the password is not. Passwords are also stored unencrypted in
  this case, as the password used to encrypt them can change underneath
  us.
- configure and Makefile changes for the backend objects

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