pam_krb5: Stitch pam-krb5 plumbing into libpam

The eyeire.org pam-krb5 supports MIT KRB5 and Heimdal. FreeBSD will use
it to implement pam_krb5 for MIT KRB5. The existing libpam pam_krb5
only supports Heimdal and therefore cannot be used with the MIT KRB5
import.

Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D50696