Fix alert callback in the QUIC layer

Fix alert callback in the QUIC layer

Only close_notify and user_cancelled are warning alerts. All others
should be fatal. In order for the lower layers to behave correctly,
the return code for fatal alerts needs to be TLS13_IO_ALERT instead
of TLS13_IO_SUCCESS.

Failure to signal handshake failure in the public API led to a crash
in HAProxy when forcing the tls cipher to TLS_AES_128_CCM_SHA256 as
found by haproxyfred while investigating
https://github.com/haproxy/haproxy/issues/2569

Kenjiro Nakayama found misbehavior of ngtcp2-based servers, wrote a
similar patch and tested this version.

Fixes https://github.com/libressl/portable/issues/1093

ok jsing