Add bind trigger for chanset

[vanosg]

Add a bind or bind event to cover changing a channel setting. Ie, .chanset #channel +foo activates a bind for the foo channel setting.

For discussion: add to bind EVNT, or a new bind? I don't think this fits under bind evnt as those are, so far, events that are external to Eggdrop. I don't know we need a bind dedicated to just this, but maybe there's an opportunity for a new EVENT-type bind, but focused towards internal eggdrop events? We've missed the boat on some things that could fall under this, like partyline signons/signoffs, but... whatever. Open to thoughts

[vanosg]

@thommey thoughts?

[vanosg]

While we're at it, probably want to add a bind event for cap registrations/removals

[thommey]

Chanset binds would be useful, especially for custom int/string settings where you could apply validity checking and reject the change if it's not a valid value.
If this is only needed for custom (setudef) channel settings, we could allow to specify a proc in the setudef command itself to register such handler.
That would not work for core eggdrop settings though.
A bind would work for both, so those are the 2 options I could think of. One way or another, a return 1 or error thrown should reject the setting change as invalid, then this adds a lot of value and a often requested feature.