Fix bug in bufferevent_connect on an openssl bufferevent that already had an fd

Fix bug in bufferevent_connect on an openssl bufferevent that already had an fd

The problem was that we were using openssl's BIO code's shutdown flag
whenever BEV_OPT_CLOSE_ON_FREE was set.  This made the BIO close the
socket when it was freed... but it would be freed whenever we did a
setfd on the bufferevent_openssl, even the no-op setfd in
bufferevent_connect.

So instead, we just set the shutdown flag to 0, and handle closing the
fd ourselves.

Spotted by Linus Nordberg