irc: display target of notice when it is displayed in channel

With the new option irc.look.notice_welcome_redirect, a private notice could
have a channel at beginning of message (with format "[#channel]") and then be
displayed on the channel, without difference with a real channel notice
(received by all nicks in channel).

For example with these commands:

  /notice nick2 [#channel] private notice
  /notice #channel notice for whole channel

Old display was (on receiver side, in #channel):

  -- | Notice(nick1): private notice
  -- | Notice(nick1): notice for whole channel

New display is:

  -- | Notice(nick1) -> nick2: private notice
  -- | Notice(nick1) -> #channel: notice for whole channel