Default to using named curve parameter encoding

Default to using named curve parameter encoding

The pre-OpenSSL 1.1.0 default was to use explicit curve parameter
encoding. Most applications want to use named curve parameter encoding
and have to opt into this explicitly.

Stephen Henson changed this default in OpenSSL commit 86f300d3 6 years
ago and provided a new OPENSSL_EC_EXPLICIT_CURVE define to opt back into
the old default.  According to Debian's codesearch, no application
currently does this, which indicates that we currently have a bad default.

In the future it is more likely that applications expect the new
default, so we follow OpenSSL to avoid problems.

Prompted by schwarze who noted that OPENSSL_EC_EXPLICIT_CURVE is missing.

ok beck inoguchi jsing