[ruby/openssl] pkey: restore support for decoding "openssl ecparam -genkey" output

Scan through the input for a private key, then fallback to generic
decoder.

OpenSSL 3.0's OSSL_DECODER supports encoded key parameters. The PEM
header "-----BEGIN EC PARAMETERS-----" is used by one of such encoding
formats. While this is useful for OpenSSL::PKey::PKey, an edge case has
been discovered.

The openssl CLI command line "openssl ecparam -genkey" prints two PEM
blocks in a row, one for EC parameters and another for the private key.
Feeding the whole output into OSSL_DECODER results in only the first PEM
block, the key parameters, being decoded. Previously, ruby/openssl did
not support decoding key parameters and it would decode the private key
PEM block instead.

While the new behavior is technically correct, "openssl ecparam -genkey"
is so widely used that ruby/openssl does not want to break existing
applications.

Fixes https://github.com/ruby/openssl/pull/535

https://github.com/ruby/openssl/commit/d486c82833

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