Add support for reading ECDSA PUTTY key files

when /\Aecdsa-sha2-(nistp(?:256|384|521))\z/
            curve = OPENSSL_CURVES[$1]

            # jruby-openssl doesn't include an EC class (version 0.9.16)
            ec_class = (::OpenSSL::PKey::EC rescue raise ArgumentError, "Unsupported algorithm: #{ppk.algorithm}")

            ec_class.new(curve).tap do |pkey|
              _, _, point = Util.ssh_unpack(ppk.public_blob, :string, :string, :mpint)
              pkey.public_key = ::OpenSSL::PKey::EC::Point.new(pkey.group, point)
              pkey.private_key = Util.ssh_unpack(ppk.private_blob, :mpint).first
            end