kdc: Add restart option

Add a new kdc_restart rc variable that manages kdc (or krb5kdc) under
daemon(8). This automatically restarts the kdc should it fail, i.e.
when it's configured to use LDAP as a backend and cannot connect to its
LDAP directory.

Set kdc_restart="YES" to auto restart kdc on abnormal termination.

Set kdc_restart_delay="N" to the number of seconds to delay before
restarting the kdc. The daemon(8) default seconds applies when not set.

Reported by: Lexi Winter <lexi.freebsd@le-fay.org>
PR: 278395
Differential Revision: https://reviews.freebsd.org/D44898