rc: Set var_run_enable to enable by default

This will load/save the /var/run directories at boot/shutdown if
and only if /var/run/ is a tmpfs mount so it is a win for tmpfs
users and a no-op for everyone else.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D44097
Sponsored by: Beckhoff Automation GmbH & Co. KG