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(' ')
core_ison Use iterators