tlog: clean up messages from the go-fuse library

Drop the date and add the "go-fuse: " prefix so you can see
where the message is coming from.

Before:
  Jun 27 09:03:15 brikett gocryptfs[4150]: 2016/06/27 09:03:15 Unimplemented opcode INTERRUPT
After:
  Jun 27 09:10:58 brikett gocryptfs[4961]: go-fuse: Unimplemented opcode INTERRUPT

internal/tlog/log_go1.5.go

@@ -24,6 +24,9 @@ func SwitchLoggerToSyslog(p syslog.Priority) {
 	if err != nil {
 		Warn.Printf("SwitchLoggerToSyslog: %v", err)
 	} else {
+		log.SetPrefix("go-fuse: ")
+		// Disable printing the timestamp, syslog already provides that
+		log.SetFlags(0)
 		log.SetOutput(w)
 	}
 }