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

Deal with initial_scid, retry_scid, and original_dcid.  We need to set
and get QUIC transport parameters timely manner.  Therefore, transport
parameter operations are now done by ngtcp2_crypto library.
Applications do not need to do set it manually.  Added several
callback function implementations to ngtcp2_crypto library which can
be directly passed to ngtcp2_callbacks: client_initial, recv_retry,
and recv_client_initial.  Installation of initial packet number space
encryption keys is now handled by ngtcp2_crypto library.  They are
done in ngtcp2_crypto_client_initial_cb, ngtcp2_crypto_recv_retry_cb
and ngtcp2_crypto_recv_client_initial_cb.

Until mainline support of QUIC are present in OpenSSL and GnuTLS, be
sure to test whether the found library versions are the patched ones
since the version number check is insufficient. This fixes the default
build on a system where a new GnuTLS is present without the patches from
the GnuTLS tmp-quic branch.

Add new ENABLE_OPENSSL and ENABLE_GNUTLS CMake options to allow
explicitly disabling these libraries to mirror the --without-openssl and
the --without-gnutls functionality of autotools.

Partially addresses https://github.com/ngtcp2/ngtcp2/issues/238

The return value from this function is not used.

examples/server accepts 2 new query parameters.

- u query parameter takes urgency value [0, 7], inclusive (e.g., u=0).

- i query parameter takes 0 or 1, and they present non-incremental and
  incremental respectively (e.g., i=1).  Note that this is not
  sh-boolean.

The server overrides stream priority with these parameters.

This reverts commit 1e2c09f8.

examples/server accepts 2 new query parameters.

- u query parameter takes urgency value [0, 7], inclusive (e.g., u=0).

- i query parameter takes 0 or 1, and they present non-incremental and
  incremental respectively (e.g., i=1).  Note that this is not
  sh-boolean.

The server overrides stream priority with these parameters.

Clarify the behaviour of ngtcp2_conn_writev_stream with
NGTCP2_WRITE_STREAM_FLAG_MORE.

This commit separates key installation function into 2 and let TLS
stack install rx and tx keys separately for handshake and 1RTT keys.
This change is aligned to the new BoringSSL API and GnuTLS API.  I
expect that OpenSSL will follow this change.  It also removes side
argument from crypto API if it can be inferred by
ngtcp2_conn_is_server.

This reverts commit 48ba0b8d.

This commit adds special handling for the very first Initial packet
which is corrupted and discarded.  If server receives such packet, it
has to drop connection silently without sending any CONNECTION_CLOSE.
Then valid packet might come later.  Keeping connection state is
harmful because corrupted packet has wrong SCID, which drops later
valid packet because mismatched SCID.  New error code
NGTCP2_ERR_DROP_CONN is added which tells server application to drop
connection silently.

Clarify the behaviour of ngtcp2_conN-writev_stream with
NGTCP2_WRITE_STREAM_FLAG_MORE.