Allow Monitor-all to ping more than one interface

[dmshimself]

I had to disable monitor-all from checking my pfsense firewall VM because the first network monitor-all found could not be pinged. The second can be, but monitor-all I think only looks at the first. The first network is my WAN, so I wouldn't want to allow pings to that to succeed and modifying the order of the pfsense network cards would be more risk than I want to take at the moment.

I wondered if a useful enhancement would be to have monitor-all check all network interfaces on VM/LXCs and consider a machine to be OK if a ping on any of them works and to fail if the ping fails on all.