Network - Use isLoopback API in place of existing localhost checks

Network - Use isLoopback API in place of existing localhost checks

Various places in QtNetwork checked for Localhost or LocalHostIPv6,
i.e. 127.0.0.1 or ::1. By using the isLoopback API, other loopback
addresses are treated the same way (e.g. 127.0.0.2 and ::ffff:127.0.0.1)

Task-number: QTBUG-22246
Change-Id: I46f55630d8646fd68034a509969a0b7cb72ca77c
Reviewed-by: Thiago Macieira (Intel) <thiago.macieira@intel.com>