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

Update ngtcp2_recv_crypto_data documentation

Use closure for scid and dcid generation

Remove unused parts of Buffer in example

Use char instead of string for \n in client.cc

This commit removes the include of ngtcp2_macro.h as it does not seem to
be used.

This commit contains a suggestion for improving ngtcp2_recv_crypto_data
documentation.

This commit suggests using a closure for the generation of ngtcp2_cids
in Client::init. The motivation for this is to reduce some minor code
duplication.

Remove unused parts of Buffer in example

A subsequent PR can further simplify `Buffer` as `begin` is now always equal to `head` and `size()` is always equal to `bufsize()`.

This commit updates write_transport_params in client.cc to use a
character '\n', instead of string "\n".

Remove unused ngtcp2_list_remove macro

Use OpenSSL return values for client_hello_cb

Remove unneeded assignments

This commit updates the client_hello_cb to use return values specified
in https://www.openssl.org/docs/man1.1.1/man3/
SSL_CTX_set_client_hello_cb.html.

Add missing dash to quiet option comment

Remove client -i --interactive option from README

Remove bandwidth measurement because it is now not used.

This commit removes the -i, --interactive option for the client example
which was removed in Commit 9ea17af6
("h3 client and server").

When SSL_dl_handshake() returns either an SSL_ERROR_WANT_CLIENT_HELLO_CB
or SSL_ERROR_WANT_X509_LOOKUP, return specific error codes rather than
-1 so that those conditions can be differentiated and handled
appropriately