Remove linebreaks from libotr messages to avoid sending random IRC commands.

When OTR session is initiated by sending ?OTR? message, Unknown command
errors are printed on Irssi status because only the first line gets sent
as PRIVMSG and the human-readable HTML-formattad instruction message gets
spit out as raw commands to the IRC server.

This patch works around the problem by converting any line break characters
(not allowed by IRC protocol) into spaces, thus also avoiding possible
security issues with any other unexpected output from libotr even if this
particular message string is fixed (like I intend to do in my next patch).
Detect the libotr-emitted HTML-formatted init string and replace it with a
description customized for IRC and irssi-otr.