stroke: Dynamically resize stroke messages

The maximum size of a stroke message is currently 64k due to the 2 byte
length field.

Fixes #943.

src/stroke/stroke_msg.h

@@ -1,11 +1,5 @@
-/**
- * @file stroke_msg.h
- *
- * @brief Definition of stroke_msg_t.
- *
- */
-
 /*
+ * Copyright (C) 2015 Tobias Brunner
  * Copyright (C) 2006 Martin Willi
  * Hochschule fuer Technik Rapperswil
  *
@@ -32,7 +26,10 @@
  */
 #define STROKE_SOCKET IPSEC_PIDDIR "/charon.ctl"
 
-#define STROKE_BUF_LEN		4096
+/**
+ * Number of bytes by which the buffer is increased as needed
+ */
+#define STROKE_BUF_LEN_INC     1024
 
 typedef enum list_flag_t list_flag_t;
 
@@ -371,7 +368,10 @@ struct stroke_msg_t {
 			char *name;
 		} counters;
 	};
-	char buffer[STROKE_BUF_LEN];
+	/* length of the string buffer */
+	u_int16_t buflen;
+	/* string buffer */
+	char buffer[];
 };
 
 #endif /* STROKE_MSG_H_ */