POD: stop abusing comment

POD: stop abusing comment

OpenSSL uses some POD directives masquerading as 'comment'
('=for comment' etc).  This is abusive and confusing.  Instead, we use
our own keyword.

    =for openssl whatever

    =begin openssl

    whatever

    =end openssl

(we have never used the multiline form, but might start one day)

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10048)