Turn the proxy code into a library

Allow other go programs to easily import the snowflake proxy library and
start/stop a snowflake proxy.

proxy/main.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"flag"
+	"io"
+	"log"
+	"os"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/safelog"
+	"git.torproject.org/pluggable-transports/snowflake.git/proxy/lib"
+)
+
+func main() {
+	capacity := flag.Int("capacity", 10, "maximum concurrent clients")
+	stunURL := flag.String("stun", snowflake.DefaultSTUNURL, "broker URL")
+	logFilename := flag.String("log", "", "log filename")
+	rawBrokerURL := flag.String("broker", snowflake.DefaultBrokerURL, "broker URL")
+	unsafeLogging := flag.Bool("unsafe-logging", false, "prevent logs from being scrubbed")
+	keepLocalAddresses := flag.Bool("keep-local-addresses", false, "keep local LAN address ICE candidates")
+	relayURL := flag.String("relay", snowflake.DefaultRelayURL, "websocket relay URL")
+
+	flag.Parse()
+
+	sf := snowflake.SnowflakeProxy{
+		Capacity:           uint(*capacity),
+		StunURL:            *stunURL,
+		RawBrokerURL:       *rawBrokerURL,
+		KeepLocalAddresses: *keepLocalAddresses,
+		RelayURL:           *relayURL,
+		LogOutput:          os.Stderr,
+	}
+
+	if *logFilename != "" {
+		f, err := os.OpenFile(*logFilename, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		sf.LogOutput = io.MultiWriter(os.Stderr, f)
+	}
+	if *unsafeLogging {
+		log.SetOutput(sf.LogOutput)
+	} else {
+		log.SetOutput(&safelog.LogScrubber{Output: sf.LogOutput})
+	}
+
+	sf.Start()
+}