Conditionally install Kerberos rc files based on MK_KERBEROS_SCRIPTS
instead of MK_KERBEROS. The reason for this change is some users
prefer to build FreeBSD WITHOUT_KERBEROS, wanting to retain the
Kerberos rc scripts to start/stop MIT Kerberos or Heimdal from ports.

PR: 197337
Reported by: Adam McDougall <ebay at looksharp.net>
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D24252