ircd: enforce unique nicknames (433), announce renames, fold case

NICK overwrote the nick with whatever was asked for, so two clients could
both be "alice": /NAMES listed her twice and a private message only ever
reached whichever slot came first in the table.

- A nick already held by another client is refused with 433
  ERR_NICKNAMEINUSE and the client keeps the one it had. Unregistered
  clients simply stay unregistered until they pick a free one, which is what
  every client already expects.
- A NICK from a registered client is a rename: it is announced to the client
  and to its channel, so nobody's user list goes stale.
- IRC names fold case, so "Alice" is "alice" - for the uniqueness check, for
  PRIVMSG by nick, and for channels ("#GB" is "#gb"). One irceq() replaces
  the exact-match compare that was used for all three.

gbos's own irc(1) already retries 433 by appending '_', so two Game Boys with
the default nick now sort themselves out: verified with a GB running ircd, a
laptop client holding "sl0pboy", and a second GB arriving as "sl0pboy_" -
both in #gbos, messages crossing both ways.

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