KEYS: Allow the public_key struct to hold a private key [ver #2]
Put a flag in the public_key struct to indicate if the structure is holding
a private key. The private key must be held ASN.1 encoded in the format
specified in RFC 3447 A.1.2. This is the form required by crypto/rsa.c.
The software encryption subtype's verification and query functions then
need to select the appropriate crypto function to set the key.