Check existing recipients before trying to add a new one

[dominikschulz]

This should avoid bad suprises when adding new recipients.

• Manually test this
• Add a nice error message
• Add a test case for this
• Add a warning when there is such a thing, and proper documentation about what is possible to do in that case (namely: extend the private key expiration date OR remove the expired key and add a new key with a expiration date in the future)
• Add a flag to "ignore" expired keys upon re-encryption as long as there are still valid keys in the recipient list (or maybe provided we are working with a non-expired subkey related to the same master key as the expired key?)
• Make sure we cannot add or create or generate new secrets if we have an expired key: Warn the user they need to add a new non-expired key and remove the expired key.

Some of these are from @AnomalRoil in #1918 (comment)

Fixes #1918

RELEASE_NOTES=[ENHANCEMENT] Check recipients before adding a new one.

Signed-off-by: Dominik Schulz dominik.schulz@gauner.org