MINOR: htx: Add the start-line offset for the HTX message in the HTX structure

If there is no start-line, this offset is set to -1. Otherwise, it is the
relative address where the start-line is stored in the data block. When the
start-line is added, replaced or removed, this offset is updated accordingly. On
remove, if the start-line is no set and if the next block is a start-line, the
offset is updated. Finally, when an HTX structure is defragmented, the offset is
also updated accordingly.
diff --git a/include/types/htx.h b/include/types/htx.h
index c709619..6d64c13 100644
--- a/include/types/htx.h
+++ b/include/types/htx.h
@@ -152,6 +152,9 @@
 	uint64_t extra;  /* known bytes amount remaining to receive */
 	uint32_t flags;  /* HTX_FL_* */
 
+	int32_t sl_off; /* Offset of the start-line of the HTTP message relatively to the beginning the
+			   data block. -1 if unset */
+
 	struct htx_blk blocks[0]; /* Blocks representing the HTTP message itself */
 };