Stop overwriting client.ip.

This is a major/syntax breaking change.

Overwriting client.ip is inherently dangerous, since it is only
set by Varnish when TCP connect happens.

Any connection from a forward proxy that coalesces multiple clients
onto a single TCP connection, where some were missing, would leak
client IP adress between requests.

Redesign this based on Dag Haavi Finstad's work in Varnish Cache Plus,
using the (somewhat less used) IP datatype in VCL instead.