Rework channel mode handling

Rework channel mode handling

Incoming MODE processing is split into a parsing step and an execution
step, instead of a mode's effector function being involved in its own
parsing. Modes can no longer use custom logic to control their parsing,
and instead supply a combination of CHM_* flags to the parser. As a
result, we know before we try to effect any mode changes what all of
them will be.

The reauthorize hack for override is no longer necessary. A side effect
of its introduction was that `MODE #foo b x!y@z` no longer worked; in
removing it we restore that behaviour.

We gain the ability to reject various invalid inputs that:
- mutate or query unknown modes
- supply excess mode arguments
- query modes that can't be queried

In each case, whether we *should* reject it is an open question; for now
I'm rejecting the first one.