Add two ways to exclude private keys when dealing with an hx509
certificate store. One as a load option (load no private keys, never
add private keys), one as a store option (store no private keys).
This is useful for CA code so it can have a single store with the
issuer's credentials _and_ the chain for it, and copy those to a store
with the issued certificate and _not_ accidentally include the issuer's
private key.
It would be much safer still to flip the default for this flag, but that
could break out-of-tree libhx509 dependents.
e515745996