logpolicy, various: allow overriding log function

This allows sending logs from the "logpolicy" package (and associated
callees) to something other than the log package. The behaviour for
tailscaled remains the same, passing in log.Printf

Updates #8249

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie1d43b75fa7281933d9225bffd388462c08a5f31

wgengine/netlog/logger.go

@@ -101,7 +101,7 @@ func (nl *Logger) Startup(nodeID tailcfg.StableNodeID, nodeLogID, domainLogID lo
 	}
 
 	// Startup a log stream to Tailscale's logging service.
-	httpc := &http.Client{Transport: logpolicy.NewLogtailTransport(logtail.DefaultHost, netMon)}
+	httpc := &http.Client{Transport: logpolicy.NewLogtailTransport(logtail.DefaultHost, netMon, nl.logger.Logf)}
 	if testClient != nil {
 		httpc = testClient
 	}