url: default conn->port to the same as conn->remote_port

... so that it has a sensible value when ConnectionExists() is called which
needs it set to differentiate host "bundles" correctly on port number!

Also, make conncache:hashkey() use correct port for bundles that are proxy vs
host connections.

Probably a regression from 7.62.0

Reported-by: Tom van der Woerdt
Fixes #3956
Closes #3957