configure: disable ssl2/ssl3 by default

PR-URL: https://github.com/joyent/node/pull/8551
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
doc: document why SSL2/SSL3 is disabled

PR-URL: https://github.com/joyent/node/pull/8551
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>