periodic: set _localbase in periodic scripts when not set in main script

[otis]

Summary

Set _localalbase for usage in periodic scripts in case it has not
already been set from within periodic(8) main script.

Reported by: gahr, otis

[freebsd_oldach.net]

Works for me!

However, this addition essentially is a duplication of the _localbase logic already present in /usr/sbin/periodic, which is (a) taking _localbase from sysctl and (b) providing a fall-back default just in case. As periodic(8), in some sense, implements a scripting facility similar to rc(8), my feeling is that it's more adequate to implement it along rc(8)'s lines, which is (a) setting _localbase from the sysctl in the main script /etc/rc, and (b) supplying a default for _localbase in /etc/defaults/rc.conf as fallback. Admittedly that's a bit of nitpicking to some extent.