Receive 'self messages' in the right query window

Original patch by `hondza <sedaj2@gmail.com>`, from [FS#833][fs]. I applied several needed style changes, and rebased to current HEAD.

This implements the IRCv3.2 [self-message][] extension partially (we can't announce its support through CAP yet). This is also the format used by the [privmsg][] znc module, and is already implemented by [several other clients][privmsg].

----

Simple standalone testing script:

```
cat > input <<EOF
:localhost 001 you :welcome
:someone!someone@localhost PRIVMSG you :message sent from someone
:you!you@localhost PRIVMSG someone :own message, to someone
EOF

cat input - | nc -l -p 6667
```

And connect irssi to localhost:6667, it should show the two messages in the same query window.

[fs]: http://bugs.irssi.org/index.php?do=details&task_id=833
[self-message]: https://github.com/ircv3/ircv3-specifications/blob/master/extensions/self-message-3.2.md
[privmsg]: http://wiki.znc.in/Privmsg