[MEDIUM] http: add options to ignore invalid header names

Sometimes it is required to let invalid requests pass because
applications sometimes take time to be fixed and other servers
do not care. Thus we provide two new options :

     option accept-invalid-http-request  (for the frontend)
     option accept-invalid-http-response (for the backend)

When those options are set, invalid requests or responses do
not cause a 403/502 error to be generated.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 7313e25..c0350d2 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -231,6 +231,7 @@
 		} st;                          /* status line : field, length */
 	} sl;                                  /* start line */
 	unsigned long long hdr_content_len;    /* cache for parsed header value */
+	int err_pos;                           /* err handling: -2=block, -1=pass, 0+=detected */
 };
 
 /* This is an HTTP transaction. It contains both a request message and a