auth_krb5: use the modern function calls to get the Kerberos error

[Julien ÉLIE]

auth_krb5 needs to be ported to the modern function calls to get the Kerberos error instead of using com_err, which will eliminate the need for including com_err.h or et/com_err.h which are no longer part of the official Kerberos API. This requires a replacement for krb5_{get,free}_error_message on older systems. rra-c-util does this in portable/krb5.h and portable/krb5-extra.c (along with a ton of other Kerberos portability stuff). It would be possible to add a subset of that logic, although it will require some surgery to fit into the INN compilation framework.

INN's configure will then no longer have to clear KRB5_LIBS if neither et/com_err.h nor com_err.h could be found and --with-krb5 was not given.

[Julien ÉLIE]

Done in commits [10252] & [10253] by Russ Allbery.