New custom channel mode API allowing reloading such modules.

New custom channel mode API allowing reloading such modules.
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.