Receive 'self messages' in the right query window

Original patch by hondza <sedaj2@gmail.com>, from FS#833. 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.
Merge pull request #162 from dequis/self-messages

Receive 'self messages' in the right query window