Add new authentication method EXTERNAL which relies on Apache authentication.

This method will use whatever username is provided in REMOTE_USER. This
means that e.g. HTTP Basic Auth is possible, but also any of the authentication
modules that Apache supports, beit CAS, SAML/Shibboleth, LDAP, etc, as long
as it returns a username that's in the users table.
Remove empty lines, fix typo.
Add reference in installation to setting up chroot.