Compared to the previous implementation based on OpenSSL, the new implementation
has these advantages:
1. Strict and reliable detection of invalid strings, including strings with
less than 4 bytes of garbage at the end;
2. Allows to use Base64 strings without '=' padding.
To support TLS sessions, Unit uses the OpenSSL built-in session cache; the
cache_size option defines the number sessions to store. To disable the feather,
the option must be zero.
To perform various configuration operations on SSL_CTX, OpenSSL provides
SSL_CONF_cmd(). Specifically, to configure ciphers for a listener,
"CipherString" and "Ciphersuites" file commands are used:
https://www.openssl.org/docs/man1.1.1/man3/SSL_CONF_cmd.html
This feature can be configured in the "tls/conf_commands" section.
A crash was caused by an incorrect timer handler nxt_h1p_idle_timeout() if
SSL_shutdown() returned SSL_ERROR_WANT_READ/SSL_ERROR_WANT_WRITE.
The flag SSL_RECEIVED_SHUTDOWN is used to avoid getting SSL_ERROR_WANT_READ, so
the server won't wait for a close notification from a client.
For SSL_ERROR_WANT_WRITE, a correct timer handler is set up.
The certificate is selected by matching the arriving SNI to the common name and
the alternatives names. If no certificate matches the name, the first bundle in
the array is chosen.
