Support for dynamic views.

Support for dynamic views.

* Two new configuration commands: 'field' and 'view'.
* A new configuration variable: 'preserve_fields'.
* In absence of user configuration, abook behaves and displays mostly like
  previous releases.
* More abstraction for database access. db_field_get(), db_field_put(), etc.
* Functions for handling lists of strings (abook_list)
* Functions for conversting abook_lists to CSV and vice-versa.
* Arbitrary number of email addresses internally. Still limited to 9 for
  practical reasons with the UI.
* Up to 35 fields per view.
* Escape as a modifier key in the editor.
* Addresses can be rolled both directions (esc-r).
* Fixed html produced by export filter.
* Updated abookrc man page and sample.abookrc accordingly.