It passes it to each parser in turn till one says it can parse it. It's not
ideal, but it seems to work - so far. Better would be to annotate it in some
way. I have considered annotating the type field so that the payload doesn't
have to have it added:
keyctl padd asymmetric.x509 "" @s </tmp/foo.x509
keyctl padd asymmetric.pkcs#8 ...
keyctl padd asymmetric.tpm ...
However, this doesn't work with "keyctl update" or "keyctl instantiate".