v3: Rework Armor checksum handling

[lubux]

GopenPGP v3 did not produce any armor checksum as recommended by the crypto refresh. Unfortunately, a popular OpenPGP library fails to parse armored messages without a checksum in certain scenarios.

This MR adds armor checksums back per default, but tries to avoid them when generating crypto refresh messages. i.e., generated by v6 keys.

In GopenPGP v3, armor checksums were not produced, following recommendations from the OpenPGP crypto refresh. Unfortunately, a widely used OpenPGP library fails to parse armored messages that lack a checksum in certain scenarios although they should be optional according to the official RFC.

This merge request (MR) reinstates armor checksums by default to ensure compatibility. However, it tries to omit the checksums when generating messages for the crypto-refresh, i.e., those generated with v6 keys.

Changes:

• API to armor data with the option to remove the checksum
• All armor functions append a checksum per default for compatibility with certain libraries although the crypto-refresh advises not to.
• Encryption and Sign handle now append a checksum when armoring. If the produced OpenPGP packets are crypto-refresh packets, the checksum is not appended as mandated by the crypto-refresh.

[lubux]

Thanks for the review @twiss . I addressed the comments.