Fix a bug in `openssl s_server -WWW` where it would attempt to invoke
`SSL_sendfile` if `-ktls -sendfile` was passed on the command line, even
if KTLS has not actually been enabled, for example because it is not
supported by the host. Since `SSL_sendfile` is only supported when KTLS
is actually being used, this resulted in a failure to serve requests.

Fixes #17503.
Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17788)