Add network-specific config for cert validation

Added the following two network-specific configuration options that can
be changed via controlpanel or webadmin:

* TrustAllCerts: Will trust ALL certificates when enabled, effectively
  disabling TLS certificate validation.
  Default value: false
* TrustPKI: Whether or not to trust PKI-valid certificates. Setting this
  to false will make znc trust only trusted certificates added by the
  user.
  Default value: true

With default values, behavior is exactly the same as before.

This is based on the work of Roelf Wichertjes. See YourBNC/znc@5c747598.

See znc/znc#866.

include/znc/IRCNetwork.h

@@ -266,6 +266,12 @@ class CIRCNetwork {
         m_uJoinDelay = uJoinDelay;
     }
 
+    void SetTrustAllCerts(const bool bTrustAll = false) { m_bTrustAllCerts = bTrustAll; }
+    bool GetTrustAllCerts() const { return m_bTrustAllCerts; }
+
+    void SetTrustPKI(const bool bTrustPKI = true) { m_bTrustPKI = bTrustPKI; }
+    bool GetTrustPKI() const { return m_bTrustPKI; }
+
     unsigned long long BytesRead() const { return m_uBytesRead; }
     unsigned long long BytesWritten() const { return m_uBytesWritten; }
 
@@ -305,6 +311,8 @@ class CIRCNetwork {
     CString m_sChanPrefixes;
 
     bool m_bIRCConnectEnabled;
+    bool m_bTrustAllCerts;
+    bool m_bTrustPKI;
     CString m_sIRCServer;
     std::vector<CServer*> m_vServers;
     size_t m_uServerIdx;  ///< Index in m_vServers of our current server + 1

include/znc/Socket.h

@@ -46,6 +46,12 @@ class CZNCSock : public Csock, public CCoreTranslationMixin {
         m_ssTrustedFingerprints = ssFPs;
     }
 
+    void SetTrustAllCerts(const bool bTrustAll = false) { m_bTrustAllCerts = bTrustAll; }
+    bool GetTrustAllCerts() const { return m_bTrustAllCerts; }
+
+    void SetTrustPKI(const bool bTrustPKI = true) { m_bTrustPKI = bTrustPKI; }
+    bool GetTrustPKI() const { return m_bTrustPKI; }
+
     void SetEncoding(const CString&);
 
     virtual CString GetRemoteIP() const { return Csock::GetRemoteIP(); }
@@ -60,6 +66,8 @@ class CZNCSock : public Csock, public CCoreTranslationMixin {
     CString m_sHostToVerifySSL;
     SCString m_ssTrustedFingerprints;
     SCString m_ssCertVerificationErrors;
+    bool m_bTrustAllCerts;
+    bool m_bTrustPKI;
 };
 
 enum EAddrType { ADDR_IPV4ONLY, ADDR_IPV6ONLY, ADDR_ALL };