Can be filtered to include only users of a certain credential level, or
users of a certain credential level or greater. Results are returned as
a linked list of plugin_user objects.
Hub function to free the memory used by the list also added.
As plugins may store the list, users may disconnect before the list is
used. Hence a copy of each user is made for the list, so that if a
plugin tries to send a message to a user who has left the hub it will be
ignored, rather than the hub trying to access memory that was free()d
when the user disconnected. The lookup function to turn a plugin_user
into a hub_user for some of the other hub functions is changed
accordingly. (I think this description is possibly more confusing than
just looking at the code, but oh well...)
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