Allow oper override to set forwards anywhere

[examknow]

Closes #61 - applies suggestions from #117

[edk0]

Thanks for the PR. So there's two reasons I suggested changing how get_channel_access is called. The first is that it's a step on the way to allowing override to work on channels you're not in in general, which might be a useful feature. But the one that's more directly relevant to this PR relates to module interaction. I think override should be aiming to report only if (a) the action was only allowed because of override, and (b) the action is going to go ahead. Using CHFL_OVERRIDE, while a bit of a hack, enables that quite nicely, since a high-priority hook (and honestly most of override should probably be using MONITOR rather than HIGHEST) can tell whether override was used (based on the alevel) and if the action is actually going to happen (based on the hook approval). While the rest of override is not doing this yet, that's the direction I think we should be going in.