This adds support for using a Heimdal-specific PKIX extension to derive
a maximum Kerberos ticket lifetime from a client's PKINIT certificate.
KDC configuration parameters:
- pkinit_max_life_from_cert_extension
- pkinit_max_life_bound
If `pkinit_max_life_from_cert_extension` is set to true then the
certificate extension or EKU will be checked.
If `pkinit_max_life_bound` is set to a positive relative time, then that
will be the upper bound of maximum Kerberos ticket lifetime derived from
these extensions.
The KDC config `pkinit_ticket_max_life_from_cert` that was added earlier
has been renamed to `pkinit_max_life_from_cert`.
See lib/hx509 and lib/krb5/krb5.conf.5.
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