When running an rc.d script outside of boot, ${_localbase} isn't set,
and rc.conf hardcodes the default value to /usr/local. Instead, try
to get the localbase from the user.localbase sysctl, and only fall
back to /usr/local if we can't for some reason.

This fixes e.g. /etc/rc.d/ldconfig when localbase has a non-default
value.