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.
Additionally, attempting to use too many modes or two times
the same letter is now detected and prevented.
Modules now request that a channel mode be added/orphaned,
instead of ugly manipulation from which that request had
to be guessed.
Slight changes are needed to modules that provide channel modes.
From the old API, one important function has been made static,
the other important function has been renamed, so loading old
modules should fail safely.
