Split long PRIVMSG and NOTICE commands into multiple messages to avoid truncation

RFC1459 specifies that each IRC command has a character limit of 512 characters, including the trailing
CRLF meaning that the entire command (including the verb [e.g. PRIVMSG] and target [e.g. #kiwiirc]) must
fit in 510 characters.

The limit is applied to both client -> IRCd and IRCd -> client messages, so we must be conservative and
leave room for the sender's prefix to ensure that the message is not truncated when the IRCd sends the
message to the target(s).

For this reason, I have chosen 350 characters as the break point.