Importprivkey shows address
Scratching an itch: make `importprivkey` output the corresponding address.
Without this PR, `importprivkey` shows no output. Because we're moving towards an "address-based" RPC interface, rather than "account-based", there's a gap when using `importprivkey` because there's no way to assign it to a specific account, but also no easy way to determine the address. This change fixes that wart.
8f16b27304