Define the secp256r1 curve

[anakinj]

This should fix #367

Would be super to try this out @m-zielinski or @obulkin. My OpenSSL version was not capable of reproducing the issue.
OpenSSL::PKey::EC.new('secp256r1') threw a OpenSSL::PKey::ECError (invalid curve name)

[obulkin]

Thanks, @anakinj. Been rather busy lately, but I'll try to validate this after the holidays.

In our case, the problematic key was pre-generated using openssl ecparam -name prime256v1. You could try to reproduce the issue this way instead, although it may still come down to have the right OpenSSL version.

[anakinj]

Im gonna merge this. The change is just adding support for that old name.