QUIC: support for setting QUIC methods with LibreSSL.

QUIC: support for setting QUIC methods with LibreSSL.

Setting QUIC methods is converted to use C99 designated initializers
for simplicity, as LibreSSL 3.6.0 has different SSL_QUIC_METHOD layout.

Additionally, only set_read_secret/set_write_secret callbacks are set.
Although they are preferred in LibreSSL over set_encryption_secrets,
better be on a safe side as LibreSSL has unexpectedly incompatible
set_encryption_secrets calling convention expressed in passing read
and write secrets split in separate calls, unlike this is documented
in old BoringSSL sources.  To avoid introducing further changes for
the old API, it is simply disabled.