zabbix_agentd: Add ARPing method for checking Internet Gateway
Since some ISP's block ICMP ping to their gateway ARPing can be an alternative.
This change adds arping alternatives for the regular (icmp) ping checks:
- ipfire.net.gateway.arping: Check if the Internet Gateway is reachable via ARPing
- ipfire.net.gateway.arpingtime: Measure the time it takes to ARPing the Internet Gateway
Signed-off-by: Robin Roevens <robin.roevens@disroot.org> Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>