When generating a new key or subkey, gpg-agent gives the same constant message,
"Please enter the passphrase to protect your new key".

This might be confusing, e.g. if run from a script or program or batch mode
gen-key, the user might not have any notice on what the program is trying to do.

gpg-agent should display some extra context in the prompt so the user can
identify what they are giving a passphrase for. For example,

• when generating a master key, we can tell the user the UID and expiry date
• when generating a subkey, we can also tell the user the master key it is

associated with, and the UIDs on it