The QUIC listener and the QUIC/ICMP dialers unconditionally set
tlsCfg.NextProtos = ["h3", "quic/v1"], discarding any user-specified
ALPN values from TLS options (via tls.options.alpn in config).
Clone the TLS config and only apply the default NextProtos when none are
already configured, matching the existing pattern in the MASQUE HTTP/3
dialer. This enables non-HTTP/3 QUIC use cases like SMB-over-QUIC which
requires ALPN "smb".
Fixesgo-gost/gost#872
ginuerzh
296d87a597