Use TLS_client_method and TLS_server_method

Use ngtcp2_conn_state as type for state field

When compiling with the flag -Wswitch-enum there are a number of
warnings like the following:
ngtcp2_conn.c:7960:11: warning:
7 enumeration values not handled in switch:
  'NGTCP2_CS_CLIENT_INITIAL',
  'NGTCP2_CS_CLIENT_WAIT_HANDSHAKE',
  'NGTCP2_CS_CLIENT_TLS_HANDSHAKE_FAILED'... [-Wswitch]
  switch (conn->state) {

This commit adds a default clause to avoid this as they are reported as
errors on the CI server and failing the build.

This commit changes the TLS_method() used by the client and server to
use more specific TLS_client_method() and TLS_server_method().

The main difference between these two are ossl_statem_accept is
undefined for the client, and ossl_statem_connect is undefined for the
server:

IMPLEMENT_tls_meth_func(TLS_ANY_VERSION, 0, 0,
                        TLS_server_method,
                        ossl_statem_accept,
                        ssl_undefined_function, TLSv1_2_enc_data)

IMPLEMENT_tls_meth_func(TLS_ANY_VERSION, 0, 0,
                        TLS_client_method,
                        ssl_undefined_function,
                        ossl_statem_connect, TLSv1_2_enc_data)

ossl_statem_accept and ossl_statem_connect are defined like this:

int ossl_statem_accept(SSL *s) {
    return state_machine(s, 1);
}
int ossl_statem_connect(SSL *s) {
    return state_machine(s, 0);
}

So they are both calling state_machine but with setting the server
parameter to 1 and 0 (for the client).

The functions ossl_statem_accept and ossl_statem_connect are only usedin
ssl/statem/statem_lib.c as far as I can tell. The tls_finish_handshake
function has the following if statement:

if (s->server) {
  ...
  s->handshake_func = ossl_statem_accept;
} else {
  ...
  s->handshake_func = ossl_statem_connect;
}

So for a server, s->handshake_func would be ossl_statem_accept which just
calls state_machine(s, 1), and or a client it would be
ossl_statem_connect which calls state_machine(s, 0).

So it looks like setting both, as in using TLS_method(), is not
neccessary and it should be safe to use TLS_client_method(),
TLS_server_method() instead.

Move template.h into server.cc

Add Wireshark configuration steps

This commit suggests updating ngtcp2_conn's state field to be of type
ngtcp2_conn_state instead of type int.

This commit adds information about configuring Wireshark for analyzing
QUIC traffic.

Add inc_len_varint function to ngtcp2_crypto.c

Move template.h include to client.cc

Move ngtcp2_range.h into ngtcp2_gaptr.c

Remove unused assert.h includes

Check mmap's return value

Remove unused ngtcp2_macro.h includes

Remove redundant Buffer::bufsize()

Previously, when mmap() failed the client example crashed deep inside ngtcp2 on trying to access address 0xffffffffffffffff. This can happen for example if `--data` specifies a directory.

This commit moves the include of template.h into client.cc as it does
not seem to be used any longer by client.h.

Commit a7d6c8c4 ("Add
ngtcp2_acked_stream_data_offset callback") removed the field streambuf_
which was using the user-defined literal `_k` from template.h:

std::array<uint8_t, 32_k> streambuf_;

The header ngtcp2_gaptr.h does not use anything from ngtcp2_range.h and
this commit suggests moving the include to ngtcp2_gaptr.c instead.

This commit contains a suggestion to add a static function to reduce
some code duplication in ngtcp2_encode_transport_params.

It's now identical to size().

Remove redundant Buffer::head

`head` always equals `begin`. A subsequent diff can further simplify Buffer by removing bufsize(), which is now identical to size().

Remove unused ngtcp2_macro include in ngtcp2_qlog