Generate compatible keys in legacy compliance modes (T7511)

[Daniel Kahn Gillmor]

The following series of 5 patches attempt to ensure that using gpg
with --compliance=rfc4880 (or earlier compliance modes), the OpenPGP
keys produced will be readable by clients that only implement the
particular legacy standard.

This is intended to address https://dev.gnupg.org/T7511

Happy to hear feedback on it!

Regards,

        --dkg