Properly split long IRC messages

This commit adds handling of long IRC messages to the core. In contrast
to the `splitlong.pl' plugin, multi-byte encoded and recoded messages
are properly split.

To allow for this, a new function has been added to the server struct:
`split_message'. `split_message' returns a string array with the message
splitted to substrings of a length that the server can handle. If a
protocol module doesn't have any limit, it can simply return a singleton
array with a copy of the message.

The `MSG' chat command now calls `split_message' before `send_message',
and emits `message own_public' / `message own_private' with each
substring, so that the string splitting will be visible in the UI.

`split_message' in the IRC module uses `recode_split' which in turn uses
iconv to properly split multi-byte encoded (and recoded) messages.