use gnutls_record_send_file() if KTLS available
For HTTP/1.x responses streaming files from disk,
gnutls_record_send_file() (when KTLS is available) might
reduce CPU usage and copying between userspace and kernel.
Note: KTLS is disabled by default in gnutls global config
"When GnuTLS is build with -–enable-ktls configuration, KTLS is disabled by default. This can be enabled by setting ktls = true in [global] section."
"The location of the default configuration file is /etc/gnutls/config, but its actual location may be overridden during compile time or at run-time using the GNUTLS_SYSTEM_PRIORITY_FILE environment variable. The file used can be queried using gnutls_get_system_config_file."
(e.g. on Fedora 37: /etc/crypto-policies/back-ends/gnutls.config)