BZ-62497: Make r.ap_auth_type writable via mod_lua

[michael-o]

This completes the option of setting the remote user by the authentication mechanism which actually verified the user.

One possible usecase is that a proxied (upstream) server performs the authentication, but the access log of HTTPd does not contain this information. The upstream server can pass this kind of information back to HTTPd and both servers will have consistent access logs.