Send a MODE request when JOINing

This kind of reverts/fixes f1cb09bd9c.

Robby found a bug with evil ChanServ which goes like this:

- You join an empty, but registered channel and the IRCd applies its default
  modes (+nt), but ZNC didn't ask for MODE, so doesn't know this.
- ChanServ applies whatever channel modes it wants to apply. This causes ZNC to
  see a mode change (e.g. +s).

The result of this is that ZNC thinks that the channel has mode +s and it will
tell every client that connects to it about this.

The fix is to send a MODE request when JOINing. To make sure that we don't
confuse clients, we block the reply.

Signed-off-by: Uli Schlachter <psychon@znc.in>

Chan.cpp

@@ -54,6 +54,7 @@ CChan::~CChan() {
 
 void CChan::Reset() {
 	m_bIsOn = false;
+	m_bModeKnown = false;
 	m_musModes.clear();
 	m_sTopic = "";
 	m_sTopicOwner = "";