curl: do not set CURLOPT_SSLENGINEDEFAULT automatically

There were bugs in the PKCS#11 engine, and fixing them triggers bugs in
OpenSSL. Just don't get involved; there's no need to be making the
engine methods the default anyway.

https://github.com/OpenSC/libp11/pull/108
https://github.com/openssl/openssl/pull/1639

Merges #1042