Enable splitting a message sent to an IRC server

When a bot message is too long, the request is truncated by the IRC server.
The limit seems to be (for the whole message) around 450 from empiric tests,
this includes as well the command and channel (so, it depends on where and what
you are sending). Looking at other FLOSS like weechat and irssi, they are
splitting at 411 for the core message itself. Using the same value.

The split happens before last available whitespace. In case a continued char
entry without any space exceed LIMIT_MESSAGE_SIZE on its own, we bindly split
it, but keep it in its own message request.

Add some tests for this as well.
Add another test for another code path

Add the case of a string exceeding LIMIT_MESSAGE_SIZE between 2 other messages.
Ensure this string is treated on its own request, but then, that other
following messages are appended to current write.

Some cosmetic change due to long lines as well.