Add 1024-bit Diffie-Hellman parameters to vconn-ssl.

Add 1024-bit Diffie-Hellman parameters to vconn-ssl.

1024-bit Diffie-Hellman is somewhat weak, but I haven't been able to
figure out how to make OpenSSL use longer Diffie-Hellman keys than that.
If this isn't available, OpenSSL doesn't let connections proceed.

This change should be reverted when we figure out how to make OpenSSL
use longer Diffie-Hellman keys.