rc.d: Fix Kerberos rc scripts

Split the Kerberos rc scripts into two sections, one for MIT Kerberos
and one for Heimdal. Don't install Heimdal-specific rc scripts (e.g.,
kfd) for the MIT case.

This does not include an OptionalObsoleteFiles change (even though one
is needed) because that will be fixed later in a different way.

Reviewed by: des (previous version)
Differential Revision: https://reviews.freebsd.org/D51830