Modification to allow an loadbalancer between browser and server about mod-auth-external HOT 4 CLOSED

This is a sound idea. I've added a version of it to the version in svn, and it 
will be included in the next release.

The one change I made is to the name. The "X-Forwarded-For" header gives a 
comma-separated list of the servers the request was forwarded to.  The first 
one will be the original client, the rest will be proxies it passed through 
before reaching the proxy whose IP is given in the IP environment variable. 
Most commonly it will be used to get the first IP address, which is the 
original client IP, not a proxy. So calling the variable PROXY seems confusing. 
I called it FORWARDS instead, plural as a reminder that this is, in general, 
going to be a list of IPs, not just a single IP.

Anyway, thanks for the contribution.

Issue 13 has been merged into this issue.

Actually, on second thought, I think that making the IP environment variable 
always return the client IP is better.  No authenticator will ever really want 
to know the whole chain of forwards.

So this change is out again.