Fix support for DragonFly BSD

The __DragonFly__ macros were introduced in issue #1546 along with a
function naming fix, but it was decided they should be handled
separately.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1765)