Don't send signature algorithms when client_version is below TLS 1.2.

Per RFC 5246,

    Note: this extension is not meaningful for TLS versions prior to 1.2.
    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.

Although second sentence would suggest that there would be no interop
problems in always offering the extension, WebRTC has reported issues
with Bouncy Castle on < TLS 1.2 ClientHellos that still include
signature_algorithms. See also
https://bugs.chromium.org/p/webrtc/issues/detail?id=4223



RT#4390

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>