Add a way to disable certain capabilities

This is a way for admins to mitigate some issues caused by caps if such issues ever arise.

E.g. add this to global level in znc.conf:

DisableClientCap = sasl
DisableServerCap = chghost
DisableServerCap = message-tags

Then these caps will be NAKed to client / not requested from server.

Note that this mechanism doesn't fully prevent a cap from being activated, e.g. one could use *send_raw module to request it from server even when disabled.

src/IRCSock.cpp

@@ -416,6 +416,9 @@ bool CIRCSock::OnCapabilityMessage(CMessage& Message) {
                 sCap = sToken.substr(0, eq);
                 sValue = sToken.substr(eq + 1);
             }
+            if (CZNC::Get().GetServerCapBlacklist().count(sCap)) {
+                continue;
+            }
             m_msCapLsValues[sCap] = sValue;
             if (OnServerCapAvailable(sCap, sValue) || mSupportedCaps.count(sCap)) {
                 m_ssPendingCaps.insert(sCap);