Added Handler.confirm_keys to replace confirm_trust_keys

Added Handler.confirm_keys to replace confirm_trust_keys

This method takes a PendingFeed object rather than separate arguments, allowing
extra information to be provided without changing the API in future. It also takes
a function for fetching extra information about a key.

To support both legacy callers and legacy subclasses, the confirm_keys method
dispatches to confirm_import_feed for new-style classes or to confirm_trust_keys
for old-style ones. If you have a custom handler that overrides confirm_trust_keys,
you should now also override confirm_import_feed. You can continue to override
confirm_trust_keys if you want to support older callers.