Offer user the choice to change the subkeys only if there is a choice

To check if the user actually has a choice we count the number of subkeys
that are:

• not revoked (cannot be extended)
• and that do have an explicit expiration date (because those subkeys

always expire together with the primary key so that there's no choice not
to change their expiration)

• and that either have not yet expired or that expired around the same

time (+/- 1 hour) as the primary key

• GnuPG-bug-id: T6958