move derp.go to derp module

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/types/const.go

@@ -0,0 +1,16 @@
+package types
+
+import "time"
+
+const (
+	HTTPReadTimeout        = 30 * time.Second
+	HTTPShutdownTimeout    = 3 * time.Second
+	TlsALPN01ChallengeType = "TLS-ALPN-01"
+	Http01ChallengeType    = "HTTP-01"
+
+	JSONLogFormat = "json"
+	TextLogFormat = "text"
+
+	KeepAliveInterval = 60 * time.Second
+	MaxHostnameLength = 255
+)