figure out NS GROUP and NS GHOST when nickname reservation is disabled

[slingamn]

This is from a report in #oragono. In theory, you should be able to GROUP an additional nick name and then GHOST it even when nickname reservation is disabled. However, there may be up to two bugs in this functionality:

1. NS GROUP is disabled when nickname reservation is disabled
2. NS GHOST doesn't work unless the nickname-to-account index is built, and if reservation is disabled, it won't be

[slingamn]

Actually, now I'm uncertain whether we actually want these features to work when nickname reservation is disabled. The way to enable the permissive mode (no proactive enforcement, but GROUP and GHOST allowed) should probably be "nickname reservation enabled, default enforcement method optional", which currently works as expected. (This is "traditional" in some sense but I do not like it, so I don't want to encourage people to use it; I'd rather people used strict nickname enforcement.)

If it turns out we don't want these features to work with nickname reservation disabled, we should make GHOST fully disabled when nickname reservation is disabled.

[slingamn]

I think the approach described in the previous comment (no GHOST or GROUP when nickname reservation is disabled) is correct.

What I'm uncertain about now is whether to change the example oragono.yaml so it defaults to nick reservation being on, with default enforcement method optional. I'm a little torn about this because it discourages operators from choosing strict instead.