apps: support sendfile in s_server when ktls enabled

apps: support sendfile in s_server when ktls enabled

When the -WWW or -HTTP option is specified, s_server can choose
to use SSL_sendfile to transmit the file requested by client
with KTLS is enabled, taking full advantage of the performance
advantages of Kernel TLS, and adding the '-sendfile' command
line parameter to control this behavior.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11318)