fix all the warnings in our demos and make them enableable

fix all the warnings in our demos and make them enableable

Fix up the warnings in the demos and make them configurable with
enable-demos

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/24047)