Rename cfg settings "adminusers" and "admingroup" to "specialusers" and "specialgroup"

The name "adminusers" could incorrectly suggest that listing users there would make them
become admins automatically, however this is actually only true as long as DokuWiki's
"superuser" setting contains a "@group" reference and the group name matches the one
specified in "admingroup". If you put a user list in "superuser", "admingroup" would
have no effect but to put the appropriate users in that group. Therefore, it is more
consistent to name the settings "specialusers" and "specialgroup". If you leave
"superuser" at its "@admin" default and "specialgroup" at its "admin" default, things
will still work out the old way.
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