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.

include/znc/znc.h

@@ -83,6 +83,8 @@ class CZNC : private CCoreTranslationMixin {
     void ClearTrustedProxies();
     bool AddTrustedProxy(const CString& sHost);
     bool RemTrustedProxy(const CString& sHost);
+    const SCString& GetClientCapBlacklist() const { return m_ssClientCapBlacklist; }
+    const SCString& GetServerCapBlacklist() const { return m_ssServerCapBlacklist; }
     void Broadcast(const CString& sMessage, bool bAdminOnly = false,
                    CUser* pSkipUser = nullptr, CClient* pSkipClient = nullptr);
     void AddBytesRead(unsigned long long u) { m_uBytesRead += u; }
@@ -307,6 +309,8 @@ class CZNC : private CCoreTranslationMixin {
     VCString m_vsBindHosts;  // TODO: remove (deprecated in 1.7.0)
     VCString m_vsTrustedProxies;
     VCString m_vsMotd;
+    SCString m_ssClientCapBlacklist;
+    SCString m_ssServerCapBlacklist;
     CFile* m_pLockFile;
     unsigned int m_uiConnectDelay;
     unsigned int m_uiAnonIPLimit;