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.

Ruby 3.0 deprecated rb_cData with the intention to remove it in release 3.1.
This commit changes references of rb_cData to rb_cObject.  This was done so we
can support distributions that package Ruby 3.0, such as Fedora 34.

We also need to call rb_undef_alloc_func because we're no longer deriving from
rb_cData.  This prevents unnecessary allocations.

See:
https://docs.ruby-lang.org/en/3.0.0/doc/extension_rdoc.html

"It is recommended that klass derives from a special class called Data
(rb_cData) but not from Object or other ordinal classes.  If it doesn't,
you have to call rb_undef_alloc_func(klass)."