+R should only prevent unregistered users from joining, not from speaking

[slingamn]

Coming out of discussion with @ajaspers:

1. +R prevents unregistered users who are joined to the channel (because they were invited, either with INVITE or +I, or joined with SAJOIN, or because they were joined before the mode was applied) from speaking. This goes back to 0046025.
2. options for muting unregistered users #1182 gave us +M (only registered users can speak)
3. Inspircd has both of these modes, apparently with the following semantics: +R only prevents joins, and +M only prevents speaking: https://docs.inspircd.org/3/modules/services_account/
4. We can make our semantics match Inspircd's; this is a minor compatibility break but it gives channel operators a more flexible toolkit. They can set +RM to restore the old semantics.