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/znc.cpp

@@ -55,11 +55,14 @@ CZNC::CZNC()
       m_vsBindHosts(),
       m_vsTrustedProxies(),
       m_vsMotd(),
+      m_ssClientCapBlacklist(),
+      m_ssServerCapBlacklist(),
       m_pLockFile(nullptr),
       m_uiConnectDelay(5),
       m_uiAnonIPLimit(10),
       m_uiMaxBufferSize(500),
-      m_uDisabledSSLProtocols(Csock::EDP_SSL | Csock::EDP_TLSv1 | Csock::EDP_TLSv1_1),
+      m_uDisabledSSLProtocols(Csock::EDP_SSL | Csock::EDP_TLSv1 |
+                              Csock::EDP_TLSv1_1),
       m_pModules(new CModules),
       m_uBytesRead(0),
       m_uBytesWritten(0),
@@ -1185,6 +1188,17 @@ bool CZNC::LoadGlobal(CConfig& config, CString& sError) {
         AddTrustedProxy(sProxy);
     }
 
+    m_ssClientCapBlacklist.clear();
+    config.FindStringVector("disableclientcap", vsList);
+    for (const CString& sCap : vsList) {
+        m_ssClientCapBlacklist.insert(sCap);
+    }
+    m_ssServerCapBlacklist.clear();
+    config.FindStringVector("disableservercap", vsList);
+    for (const CString& sCap : vsList) {
+        m_ssServerCapBlacklist.insert(sCap);
+    }
+
     CString sVal;
     if (config.FindStringEntry("pidfile", sVal)) m_sPidFile = sVal;
     if (config.FindStringEntry("statusprefix", sVal)) m_sStatusPrefix = sVal;