ktls: add sendfile

[lrstewart]

Resolved issues:

Resolves #4171

Description of changes:

Add a wrapper method for sendfile.

There are actually multiple definitions of sendfile:

• Linux: ssize_t sendfile(int out_fd, int in_fd, off_t *_Nullable offset, size_t count);
• FreeBSD: int sendfile(int fd, int s, off_t offset, size_t nbytes, struct sf_hdtr *hdtr, off_t *sbytes, int flags);
• MacOS: int sendfile(int fd, int s, off_t offset, off_t *len, struct sf_hdtr *hdtr, int flags);

We only need to support Linux for now, since we only support ktls in Linux. However, we need to ensure that the signature we choose can work for all implementations. So I dropped any feature only supported by one implementation, like Linux's special handling of offset==NULL and the sf_hdtr field. Our signature should be as minimal as possible.

Call-outs:

Why not just let customers call sendfile directly? This would prevent us from ever updating the connection in response to a send (like if we track bytes sent) and from sending control messages like post-handshake messages.

Testing:

A little unit test that tests sendfile without ktls actually enabled. It tries to send itself :)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[lrstewart]

Related to #4187

My new S2N_LINUX_SENDFILE feature flag was failing in make (but not in cmake) with "unused variable", and so was S2N_KTLS_SUPPORTED. Probably best just to ignore unused variables, since they have nothing to do with feature support.

But if we think that's too risky, I can just fix both feature tests to not have unused variables. However, I suspect we'll keep making this mistake.