ssl: Add SSL_sendfile

This commit adds the SSL_sendfile call, which allows KTLS sockets to
transmit file using zero-copy semantics.

Signed-off-by: Boris Pismenny <borisp@mellanox.com>

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8727)