Autologin: Do NOT sanitize username

I don't know the reason why this was done initially but a username must not be changed.
Autologin: Use REMOTE_USER for authentication

It's not safe to rely on PHP_AUTH_USER and PHP_AUTH_TYPE because
PHP cgi handlers (fgcid for example) only set the REMOTE_USER environment variable
and the authentication type for negogiation methods (Kerberos for example) is neither
Basic nor Digest.
We may have to add REDIRECT_REMOTE_USER for authentication for proxy setups.
Autologin: Do not require a bogus password in the source code
Autologin: Do not use absolute `use'
Autologin: Actually set the username upon authentication

Before, when using autologin the username of the authenticated user always was the empty string.
Autologin: Fix PHPDoc