pam_ksu: Proactively address MIT KRB5 build failure

MIT KRB5 does not provide a krb5_make_principal() function. We need to
provide this ourselves for now. We provide the function for now while
MIT and Heimdal are both in the tree. When Heimdal is removed we can
inline the calls to krb5_get_default_realm() and
krb5_build_principal_va(). krb5_build_principal_va() is
deprecated in MIT KRB5. Its replacement, krb5_build_principal_alloc_va()
will be used instead at that time.

Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D50808