Fix host name retrieval during TLS certificate generation (#1935)

Use HOSTNAME if defined, otherwise invoke gethostname(3) if possible,
fallback to uname(2)

src/znc.cpp

@@ -316,7 +316,7 @@ bool CZNC::WritePemFile() {
         return false;
     }
 
-    CUtils::GenerateCert(f, "");
+    CUtils::GenerateCert(f);
     fclose(f);
 
     CUtils::PrintStatus(true);