Ignore warning alert returns from servername callback in TLSv1.3

Ignore warning alert returns from servername callback in TLSv1.3

If a servername callback returns SSL_TLSEXT_ERR_ALERT_WARNING, this
results in a fatal error in TLSv1.3 since alert levels are implicit
in the alert type and neither close_notify nor user_canceled make
sense in this context. OpenSSL chose to ignore this, so we need to
follow suit.

Found via a broken servername callback in p5-IO-Socket-SSL which
returns a Boolean instead of SSL_TLSEXT_ERR_*. This happened to
have worked before TLSv1.3 since warning alerts are often ignored.

This "fixes" sni.t and sni-verify.t in p5-IO-Socket-SSL.

ok beck jsing