RemoteUser AuthenticationMethod

[Cantor, Scott]

I've added documentation about this to the wiki. The main issue is that
you really can't use the stock handlers to support multiple methods at
once, because they have to return the result, and they can only be taught
how to return one specific value, or left to the default.

Multiple methods really requires a custom handler, and in most cases it
did anyway.

It's also a trivial matter to copy/customize the RemoteUser handler to set
the method based on a header or something like that.

-- Scott