This commit makes the hxtool ca sub-command, when invoked with
--generate-key=TYPE and --certificate-private-key=STORE, write the
private key only to the given --certificate-private-key store and not
also the --certificate=STORE.
Before this commit, invoking the hxtool ca sub-command with both,
--generate-key=TYPE and --certificate-private-key=STORE, caused the
--generate-key option to be ignored and the private key to be read from
the given store and copied to the --certificate=STORE. That was clearly
a bug and non-sensical.