auth: strip RFC4007 zone identifiers from IPv6 addresses before doing access control

If using IPv6, the request IP address might contain a '%' that the ipaddr
module that is used for IP filtering can't handle.

https://tools.ietf.org/html/rfc4007#section-11 specifies how IPv6 addresses can
have zone identifiers like trailing '%13' or '%eth0'. The zone identifier is
used to help distinguish *if* the same address should be available on multiple
interfaces. It *could* potentially have security implications in the odd case
where the same address is different on different interfaces. The IP whitelist
functionality does however not support zone filters, so there is no way users
can expect the zone to be relevant for IP filtering. We can thus safely strip
the zone index and only check for match on the other parts of the address.