Use own database tables and follow Drupal coding standards

Problem/Motivation
- The module shares the fs_authmap table with other modules and uses the fs_netforum_auth table. Uninstalling one module can break the other, and the names don't follow the Drupal convention of starting table names with the module name.
- Several global functions (to_xml(), uid_from_username(), netforum_*()) don't start with the module name and can clash with functions from other modules.
- README.txt still describes Drupal 7.

Proposed resolution
- Move data to netauth_user and netauth_authmap with an update hook.
- Rename global functions with the netauth_ prefix.
- Replace README.txt with an up-to-date README.md.