periodic: set _localbase for periodic scripts from within periodic.conf

Set _localbase variable from within /etc/defaults/periodic.conf
for use by periodic scripts.

This fixes e5d7100c09, no other functional changes intended.

Reported by: gahr, otis
Reviewed by: markj, gahr
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47486