relax signature algorithms extension in client hello

OpenSSL, even if -tls1 or -tls1_1 are provided, adds such an extension in the
Client Hello

from RFC5246: Clients MUST NOT offer it if they are offering prior versions.
However, even if clients do offer it, the rules specified in [TLSEXT]
require servers to ignore extensions they do not understand.

lib/engine.mli

@@ -67,7 +67,6 @@ type client_hello_errors = [
   | `NotSetCiphersuites of Packet.any_ciphersuite list
   | `NoSupportedCiphersuite of Packet.any_ciphersuite list
   | `NotSetExtension of Core.client_extension list
-  | `HasSignatureAlgorithmsExtension
   | `NoSignatureAlgorithmsExtension
   | `NoGoodSignatureAlgorithms of Core.signature_algorithm list
   | `NoKeyShareExtension

lib/handshake_common.ml

@@ -303,11 +303,7 @@ let client_hello_valid version (ch : client_hello) =
               | _, _, false -> Error (`NotSubsetKeyShareSupportedGroup (gs, ks)) )
         | Some x -> Error (`NoGoodSignatureAlgorithms x)
       )
-    | `SSL_3 | `TLS_1_0 | `TLS_1_1 ->
-      Option.fold
-        ~none:(Error `HasSignatureAlgorithmsExtension)
-        ~some:(fun _ -> Ok ())
-        sig_alg
+    | `SSL_3 | `TLS_1_0 | `TLS_1_1 -> Ok ()
   in
 
   let share_ciphers =

lib/state.ml

@@ -237,7 +237,6 @@ type client_hello_errors = [
   | `NotSetCiphersuites of Packet.any_ciphersuite list
   | `NoSupportedCiphersuite of Packet.any_ciphersuite list
   | `NotSetExtension of client_extension list
-  | `HasSignatureAlgorithmsExtension
   | `NoSignatureAlgorithmsExtension
   | `NoGoodSignatureAlgorithms of signature_algorithm list
   | `NoKeyShareExtension