Reviewed-by: NRichard Levitte <levitte@openssl.org>

If no serverinfo extension is found in some cases, do not abort the handshake,
but simply omit/skip that extension.

Check for already-registered serverinfo callbacks during serverinfo
registration.

Update SSL_CTX_use_serverinfo() documentation to mention the need to reload the
same serverinfo per certificate, for servers with multiple server certificates.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>

Signed-off-by: NRich Salz <rsalz@akamai.com>
Reviewed-by: NMatt Caswell <matt@openssl.org>

serverinfo rejects non-empty extensions.

Omit extension if no relevant serverinfo data.

Improve error-handling in serverinfo callback.

Cosmetic cleanups.

s_client documentation.

s_server documentation.

SSL_CTX_serverinfo documentation.

Cleaup -1 and NULL callback handling for custom extensions, add tests.

Cleanup ssl_rsa.c serverinfo code.

Whitespace cleanup.

Improve comments in ssl.h for serverinfo.

Whitespace.

Cosmetic cleanup.

Reject non-zero-len serverinfo extensions.

Whitespace.

Make it build.