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/src/connection.c b/src/connection.c
index ca9a8ca..db869fb 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -1096,7 +1096,7 @@
 					tlv->client |= PP2_CLIENT_CERT_CONN;
 			}
 			if (srv->pp_opts & SRV_PP_V2_SSL_CN) {
-				struct chunk *cn_trash = get_trash_chunk();
+				struct buffer *cn_trash = get_trash_chunk();
 				if (ssl_sock_get_remote_common_name(remote, cn_trash) > 0) {
 					ssl_tlv_len += make_tlv(&buf[ret+ssl_tlv_len], (buf_len - ret - ssl_tlv_len), PP2_SUBTYPE_SSL_CN,
 								cn_trash->data,
@@ -1104,7 +1104,7 @@
 				}
 			}
 			if (srv->pp_opts & SRV_PP_V2_SSL_KEY_ALG) {
-				struct chunk *pkey_trash = get_trash_chunk();
+				struct buffer *pkey_trash = get_trash_chunk();
 				if (ssl_sock_get_pkey_algo(remote, pkey_trash) > 0) {
 					ssl_tlv_len += make_tlv(&buf[ret+ssl_tlv_len], (buf_len - ret - ssl_tlv_len), PP2_SUBTYPE_SSL_KEY_ALG,
 								pkey_trash->data,