Trying to port "External" login handler

[Cantor, Scott]

This seems to be working pretty well, and it turns out that even if you
mount a servlet at "/" in web.xml, you can still override other paths and
install additional servlets at nested locations, which I'm rather
surprised by, in a "Java servlet architecture is uniformly awful, so why
wouldn't it screw me here too" way.

So I ported the RemoteUserAuthServlet from V2, converted it to use the new
interface I defined, and installed at at /Authn/RemoteUser, which was the
old location used for REMOTE_USER integration.

I need to work out the relationships here, but I think I'll probably
create a dedicated flow for this that just reuses the external-flow
components but is specific to the REMOTE_USER use case so it doesn't get
too confusing.

We also have to work out whether we can create a login.jsp that actually
works for this use case (container auth via j_security_check) and our
flows, but I suspect that's doable.

-- Scott