Fixes for bug #3607 (Problems with apinger being incorrectly configured when using a PPoE interface)

This fixes two issues related to bug https://redmine.pfsense.org/issues/3607.

When using PPoE, apinger will typically be misconfigured and will fail to start.

The first issue is an incorrect mechanism used to detect the type (IPv4/IPv6) of the interface as in the PPoE case the IP is often set to "dynamic".

The second issue is a problem whereby apinger won't receive the ICMP6 replies because the source address can sometimes be incorrectly set when specifying a routable 'monitor' IPv6 address (which is often something you have to do in the UK when using PPoE as it seems most ADSL backends won't reply to an ICMP6 sent to the local link address).