re-work -debug switch handling

- re-work -debug help message text
- make -debug log every debugging information again (even all categories)
- remove unneeded fDebug checks in front of LogPrint()/qDebug(), as that
  check is done in LogPrintf() when category is != NULL (true for all
  LogPrint() calls
- remove fDebug ONLY in code which is NOT performance-critical
- harmonize addrman category name
- deprecate -debugnet usage, should be used via -debug=net and remove the
  corresponding global

src/qt/paymentrequestplus.cpp

@@ -75,8 +75,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
         digestAlgorithm = EVP_sha1();
     }
     else if (paymentRequest.pki_type() == "none") {
-        if (fDebug)
-            qDebug() << "PaymentRequestPlus::getMerchant : Payment request: pki_type == none";
+        qDebug() << "PaymentRequestPlus::getMerchant : Payment request: pki_type == none";
         return false;
     }
     else {