MAJOR: chunks: replace struct chunk with struct buffer

Now all the code used to manipulate chunks uses a struct buffer instead.
The functions are still called "chunk*", and some of them will progressively
move to the generic buffer handling code as they are cleaned up.
diff --git a/include/proto/spoe.h b/include/proto/spoe.h
index 299836b..b6c4ee2 100644
--- a/include/proto/spoe.h
+++ b/include/proto/spoe.h
@@ -164,7 +164,7 @@
 
 		case SMP_T_STR:
 		case SMP_T_BIN: {
-			struct chunk *chk = &smp->data.u.str;
+			struct buffer *chk = &smp->data.u.str;
 
 			/* Here, we need to know if the sample has already been
 			 * partially encoded. If yes, we only need to encode the
@@ -175,7 +175,7 @@
 				 * type (string or binary), the buffer length
 				 * (as a varint) and at least 1 byte of the
 				 * buffer. */
-				struct chunk *chk = &smp->data.u.str;
+				struct buffer *chk = &smp->data.u.str;
 
 				*p++ = (smp->data.type == SMP_T_STR)
 					? SPOE_DATA_T_STR