Add support for OpenSSL 3.x SSL_sendfile in mod_tls

[Castaglia]

The scope of this ticket is to add support/usage of the SSL_sendfile function, available in OpenSSL 3.x, to the mod_tls module.

The SSL_sendfile function makes use of TLS support in the kernel (where available), to increase performance by avoiding the copying of memory buffers from userland to kernel space. This is mostly beneficial for FTPS downloads; it does not benefit FTPS uploads or directory listings.

See:

• https://www.nginx.com/blog/improving-nginx-performance-with-kernel-tls/

From looking through the OpenSSL 3.x source code, it looks like they try to use KTLS automatically, where possible. If that is true, then what changes need to happen, if at all, in mod_tls? The answer is that we can be more efficient, for TLS downloads, in mod_xfer. In that module, there is already a code path for using sendfile(2), to send the entire file, when possible. And right now, mod_xfer falls back to send the data, chunk by chunk, for TLS downloads. It would be more efficient if mod_xfer knew that it could send the entire file, for TLS downloads, at once rather than chunk by chunk.

In addition, we may need to set the SSL_OP_ENABLE_KTLS option; see SSL_CTX_set_options.