smtps and proxy_v2 do not work together

[squalus]

When the smtps and proxy_v2 options are enabled for a listener, the TLS handshake rarely completes.

There can be data remaining in the io buffer after parsing the proxy header in proxy_io. Then, the TLS accept function is directed to read from the socket fd in io_start_tls. It looks like the data in the buffer is never handled, and so the SSL_accept function hangs.

OpenSMTPD 6.8.0p2 / Linux 5.10.52