[peruser] peruser keepalive support

[bert at ev6.net]

Are browsers even smart enough to use KeepAlive when different dns names resolve to a single address? Or does it just use it for subsequent requests from the same domain.
Assuming it does, that could break load balancers that direct to different back ends depending on host.
As for using different IP per site that really isn't feasible for ipv4 but I do intend to do that for ipv6 sites. Has anyone had a chance to look into the logging issue with v6?

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