Use custom PHP for username mangling/user creation

I will have to add this functionality as part of work for my current employer, so I am assigning this feature request to myself.

1. Add sections to the webserver_auth settings page for two bits of custom PHP code.
2. Add a testing tab similar to admin/user/rules/check to the webserver_auth settings page.
3. Modify webserver_auth.module to use custom PHP code from #1 above for username mangling.
4. Modify webserver_auth.module to use custom PHP code from #1 above for setting new user data.

This is needed because our production environment involves a Apache 2 server running mod_auth_kerb, which does Basic Auth against a Kerberos server. However, the $_SERVER['REMOTE_USER'] usernames resemble "smithj1@EXAMPLE.COM" — not great for a production website. We have an LDAP server available that would allow me to map "smithj1@EXAMPLE.COM" to a real name ("John Smith") and also retrieve that user's e-mail address.

To do this in webserver_auth makes more sense than doing it in core (contentious, see #102679: Add a Display Name field to core in addition to Username) or in a theme (http://drupal.org/node/122303). Rather than hard-code all of this as a module that noone else will ever use, I propose to add some flexibility to webserver_auth so other site admins can implement their own solutions according to their situations, from within the Drupal interface.