plugin: Introduced a concept of ``read groups''.

plugin: Introduced a concept of ``read groups''.

Using a new parameter added to plugin_register_complex_read(), it's now
possible to specify a group name for the registered callback. This name may
later be used to unregister *all* callbacks belonging to that group using the
newly added function plugin_unregister_read_group().

This might come in handy for plugins that register multiple read callbacks but
do not want to keep track of all of them in order to be able to unregister
them.