vici: Increase vici message length header from 16 to 32 bits

While we currently have no need for messages larger than 65KB, we should design
the protocol to be future-proof, as we plan to keep at least to lowest protocol
layer stable.

To avoid any allocation issues, we currently keep the message size limit at
512KB.

src/libcharon/plugins/vici/vici_socket.h

@@ -23,6 +23,11 @@
 
 #include <library.h>
 
+/**
+ * Maximum size of a single message exchanged.
+ */
+#define VICI_MESSAGE_SIZE_MAX (512 * 1024)
+
 typedef struct vici_socket_t vici_socket_t;
 
 /**