Change TS6 JOIN processing
- don't send out simple modes in TS6 JOIN and TS5 SJOIN when
a local user joins an existing channel
- don't send out simple modes in TS6 JOIN and TS5 SJOIN when
propagating a TS6 JOIN
- don't interpret simple modes in an incoming TS6 JOIN
This is to avoid desyncs when certain mode changes (e.g. -im)
cross with joins. A downside is that simple modes will be
more desynched when a JOIN creates a channel or lowers TS,
but that's less important.
Update the TS6 specification to include this, and clarify
that TMODE can come from a server and that MODE must be
translated into TMODE from other servers too.
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