service: use self.changed in service_enable_rcconf().

This sets the "changed" result of the module when a rc file is modified.
Previously only --check mode would show that it was going to change.

I have tested this on a FreeBSD box, and it seems to work as expected.
I believe this is a remnant from before the svc_change variable was introduced which now makes it possible to keep track of both changing the "enabled" state as well as the actual service state.