vsl: Bump SHMLOG marker for Log segments

This means an older varnishlog can no longer read logs from a live
current varnishd server, and vice versa. It used to be interesting
to use a more modern VUT to process logs for example to get better
performance or new features like generalized -E.

include/vsl_priv.h

@@ -58,7 +58,7 @@
  */
 
 struct VSL_head {
-#define VSL_HEAD_MARKER		"VSLHEAD1"	/* Incr. as version# */
+#define VSL_HEAD_MARKER		"VSLHEAD2"	/* Incr. as version# */
 	char			marker[8];
 	ssize_t			segsize;
 	unsigned		segment_n;