Clarify what RUN_ONCE returns

RUN_ONCE really just returns 0 on failure or whatever the init
function returned.  By convention, however, the init function must
return 0 on failure and 1 on success.  This needed to be clarified.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2225)