The request line sent to the book bot comes from the request-supplied
source_id and was written to the socket as-is, so a line break in it sent
extra IRC commands as the operator's nick. IRCClient._send, the single
writer, now raises IRCError on any control character (CTCP 0x01 excepted),
the download handler rejects such a request line with a clear error before
connecting, and search refuses a query that contains one.
