Use safestack.h exclusively internally.

Remove all stack headers from some includes that don't use them.

Avoid a genearic untyped stack use.

Update stack POD file to include the OPENSSL_sk_ API functions in the notes
section.  They were mentioned in the name section but not defined anywhere.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4430)