Consider renaming to support key agreement keys as well

[dlongley]

It's unclear what the name should be -- as ec-multikey seems misleading / generic as it doesn't cover, for example, ed25519/x25519 keys (which aren't supported here). The only curves that are supported or will be supported are those that work with ECDSA -- even if a P-256 / P-384 / P-256K key is to be created to only be used for key agreement (ECDH) and not ECDSA.