keyringctl:
Add `persist_basic_key()`, `persist_direct_keys()`,
`persist_certifications()` and `persist_revocations()` to allow for
dedicated writing of basic key material, direct key signatures,
per UID certificates and per UID revocations (respectively).
Change `convert_certificate()` to call the new dedicated write functions
instead of implementing the functionality.
Change `convert_certificate()` to raise on missing current_packet_key
when trying to work on signature files (this is unlikely to occur,
unless the input data is somehow broken, but it keeps the linter happy).
Change `convert_certificate()` to handle direct_keys by issuer on a
given root key (DirectKey signatures by the same issuer are combined).
Change the argparse subparser for the 'convert' command to include a
help text.
