Fix potential wraparound in conn_update_recv_rate()

This is unlikely to wraparound in the real world, but fixing it makes the code easier to fuzz.

gnutls: add stub ngtcp2_crypto_set_local_transport_params

Reported by Peter Wu in:
https://github.com/curl/curl/pull/5443#issuecomment-633138665

Remove NGTCP2_ALPN_H3 macro which should be defined by HTTP/3 layer.