curl: output warning at --verbose output for debug-enabled version

+ a libcurl warning in the debug output

Assisted-by: Jay Satiro

Ref: https://curl.se/mail/lib-2023-01/0039.html
Closes #10278

lib/multi.c

@@ -1861,6 +1861,15 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
     multistate(data, MSTATE_COMPLETED);
   }
 
+#ifdef DEBUGBUILD
+  if(!multi->warned) {
+    infof(data, "!!! WARNING !!!");
+    infof(data, "This is a debug build of libcurl, "
+          "do not use in production.");
+    multi->warned = true;
+  }
+#endif
+
   do {
     /* A "stream" here is a logical stream if the protocol can handle that
        (HTTP/2), or the full connection for older protocols */

lib/multihandle.h

@@ -170,6 +170,9 @@ struct Curl_multi {
 #endif
   BIT(dead); /* a callback returned error, everything needs to crash and
                 burn */
+#ifdef DEBUGBUILD
+  BIT(warned);                 /* true after user warned of DEBUGBUILD */
+#endif
 };
 
 #endif /* HEADER_CURL_MULTIHANDLE_H */

src/tool_help.c

@@ -151,10 +151,21 @@ void tool_help(char *category)
   free(category);
 }
 
+static bool is_debug(void)
+{
+  const char *const *builtin;
+  for(builtin = feature_names; *builtin; ++builtin)
+    if(curl_strequal("debug", *builtin))
+      return TRUE;
+  return FALSE;
+}
 
 void tool_version_info(void)
 {
   const char *const *builtin;
+  if(is_debug())
+    fprintf(stderr, "WARNING: this libcurl is Debug-enabled, "
+            "do not use in production\n\n");
 
   printf(CURL_ID "%s\n", curl_version());
 #ifdef CURL_PATCHSTAMP