Linux ktls sendfile

This commit introduces support for Linux KTLS sendfile.
Sendfile semantics require the use of a kernel TLS socket to construct the TLS
record headers, encrypt and authenticate the data.
KTLS sendfile improves performance by avoiding the copy of file data into user
space, which is required today.

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)