periodic: Honor kern.localbase

Take kern.localbase into account with fallback to /usr/local and
also allow for LOCALBASE/etc/periodic.conf (similarly to what
many other utilities do).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40435

(cherry picked from commit e5d7100c09e03e66535eee69d7e454c01e4a9d28)