lib: Accomodate FreeBSD return value for ssl connection.

On FreeBSD I see ECONNRESET being returned from check_connection_completion
in ssl_connect.  This happens before entering the SSL states, which return
EPROTO on failure.

Signed-off-by: Ed Maste <emaste@adaranet.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>