Fixed truncation of the reason when receiving a KILL message.

The KILL handler thought the message started one word later than it did, and
would cut off the first word.  This change starts the reason in the correct
place, and shaves off a leading : if one is present.

Closes #977
Closes #774
Properly support notices to prefix'd users on a channel

Instead of hardcoding this behavior for @, +, and %, fetch the list of
mode prefixes from the server's state and look for one of those instead.

Closes #977