Eliminate `conn->bits.ipv6_ip`
The bit was only correct for the first transfer using a connection. Use
`data->state.up.hostname` instead in places that need the URL hostname
in its original form.
Fix parseurlandfillconn() to not modify `data->state.up.hostname` before
copying the connection's hostname, but modify the copy instead, leaving
the URL hostname intact.
Closes#20919
