This commit adds:
- hx509_cert_init_private_key() for creating an hx509_cert object that
has just a private key
- hx509_cert_have_private_key_only() for checking whether an hx509_cert
object has just a private key
This also generalizes the get_key() internal function in hxtool, which
is tasked with reding or generating a private key for use in signing
CSRs. Now hxtool request-create can read/write private keys to/from PEM
files, not just DER files.
This is needed to support key types other than just RSA for CSRs and
certificates.
252487dfe4