feat(cli): put the network state of the host in the log journal

The journal records the network state of the host and each change of
that state. The support team reads one file to see the interfaces, the
routes, the resolvers, and the recoveries around an incident. The same
file shows the health of the query path. A change that touches only
AirDrop or virtual interfaces runs no DNS work.

cmd/cli/dns_intercept_settle.go

@@ -2,11 +2,20 @@ package cli
 
 import "github.com/Control-D-Inc/ctrld"
 
-var initializeOsResolver = ctrld.InitializeOsResolver
+var initializeOsResolver = ctrld.InitializeOsResolverWithReason
+
+// Reasons that the journal reports when the resolver set changes. Each one
+// names the code path that read the resolvers of the host.
+const (
+	osResolverReasonStart          = "start"
+	osResolverReasonWakeProbe      = "wake_probe"
+	osResolverReasonVPNSettle      = "vpn_settle"
+	osResolverReasonDelayedRecheck = "delayed_recheck"
+)
 
 func (p *prog) refreshDNSAfterVPNSettle(reason string) (routes, domainlessServers, exemptions int) {
 	mainLog.Load().Info().Msgf("DNS intercept: refreshing OS/VPN DNS route state after VPN settle (%s)", reason)
-	ns := initializeOsResolver(true)
+	ns := initializeOsResolver(true, osResolverReasonVPNSettle)
 	mainLog.Load().Debug().Msgf("DNS intercept: post-settle OS resolver nameservers: %v", ns)
 
 	if p.vpnDNS == nil {