Fix some misunderstandings in our providers' main modules

Fix some misunderstandings in our providers' main modules

This started with adding forward declarations of all provider side
interface functions, and fixing all compiler errors.

Furthermore, diminish the faulty assumption that the provider context
is and always will be just a library context.  That means adding a
teardown function in all providers that aren't necessarily built into
libcrypto.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11777)