core_list Do the HasPrivPermission() check only once, not once per chan
core_list Check whether the chan name/topic has to be Match()ed once, not once per chan
core_list Improve readability by assigning the Channel being inspected to a variable
core_ison Remove check that is always true
core_ison Don't deduplicate nicks

Deduplication makes us do pointless processing for every ISON and offers
no benefit at all to proper clients that send a nick only once
core_ison Truncate string after sending it on overflow instead of recreating it
core_ison Extract duplicated code into a function

Change append(" ") to push_back(' ')