all: add router client info detection

This commit add the ability for ctrld to gather client information,
including mac/ip/hostname, and send to Control-D server through a
config per upstream.

 - Add send_client_info upstream config.
 - Read/Watch dnsmasq leases files on supported platforms.
 - Add corresponding client info to DoH query header

All of these only apply for Control-D upstream, though.

config_internal_test.go

@@ -147,6 +147,28 @@ func TestUpstreamConfig_Init(t *testing.T) {
 				Timeout:     0,
 			},
 		},
+		{
+			"doh+doh3 with send client info set",
+			&UpstreamConfig{
+				Name:           "doh",
+				Type:           "doh",
+				Endpoint:       "https://example.com?k=v",
+				BootstrapIP:    "",
+				Domain:         "",
+				Timeout:        0,
+				SendClientInfo: ptrBool(false),
+			},
+			&UpstreamConfig{
+				Name:           "doh",
+				Type:           "doh",
+				Endpoint:       "https://example.com?k=v",
+				BootstrapIP:    "",
+				Domain:         "example.com",
+				Timeout:        0,
+				SendClientInfo: ptrBool(false),
+				u:              u2,
+			},
+		},
 	}
 
 	for _, tc := range tests {
@@ -158,3 +180,7 @@ func TestUpstreamConfig_Init(t *testing.T) {
 		})
 	}
 }
+
+func ptrBool(b bool) *bool {
+	return &b
+}