[BUG] Unhandling loopback source when sbc under NAT

[hnimminh]

trigger by #54

[hnimminh]

fix nat ignore for proxy on loopback network https://github.com/signalwire/freeswitch/pull/1719

Environment:

• Kamailio is sip proxy for FreeSWITCH, they are runing on the same in.
• Kamailio bind on loopback 127.0.0.3
• FreeSWITCH SIP bind on 127.0.0.2 (sip-ip, ext-sip-ip), RTP bind on PRIVATE_IP (rtp-ip) and PUBLIC_IP (ext-rtp-ip)
• Set the local-network-acl to rfc1918.auto or none

Step: make call from UAC (sip phone) [UAC] ---internet ---- [KAMAILIO] --- loopback ---- [FreeSWITCH] ------- [NEXTHOP]

Actual Result:

• IP address in SDP body is always PRIVATE_IP

Expected:

• IP address in SDP body is always PUBLIC_IP

Self-Check: since the NAT check logic in switch_core_media.c always include ignore if remote is loopback as below:

    return (smh->mparams->extsipip &&
            !switch_check_network_list_ip(network_ip, "loopback.auto") &&
            !switch_check_network_list_ip(network_ip, smh->mparams->local_network));

Work Around Override built in loopback.auto by:

        <list name="loopback.auto" default="deny">
          <node type="allow" cidr="127.0.0.1/32"/>
       </list>