tool: fix memory mixups

memory allocated by libcurl must be freed with curl_free() and vice versa,
memory allocated by the tool itself must be freed with curlx_free().

- dynbuf: free libcurl data with curl_free()
- tool_operate: make sure we get URL using the right memory
- tool_operhlp: free libcurl memory with curl_free()
- tool_operate: free curl_maprintf() pointer with curl_free
- var: data from curlx_base64_decode needs curlx_free
- tool_operate: fix memory juggling in etag handling
- tool_cb_hdr: fix memory area mixups
- tool_operate: another mixup in etag management
- tool_cb_hdr: more memory mixup fixes
- tool_cfgable.c: document some details
- tool_help: show global-mem-debug in -V output

Closes #21099

src/tool_help.c

@@ -356,6 +356,9 @@ void tool_version_info(void)
     size_t feat_ext_count = feature_count;
 #ifdef CURL_CA_EMBED
     ++feat_ext_count;
+#endif
+#ifdef CURL_DEBUG_GLOBAL_MEM
+    ++feat_ext_count;
 #endif
     feat_ext = curlx_malloc(sizeof(*feature_names) * (feat_ext_count + 1));
     if(feat_ext) {
@@ -364,6 +367,9 @@ void tool_version_info(void)
       feat_ext_count = feature_count;
 #ifdef CURL_CA_EMBED
       feat_ext[feat_ext_count++] = "CAcert";
+#endif
+#ifdef CURL_DEBUG_GLOBAL_MEM
+      feat_ext[feat_ext_count++] = "global-mem-debug";
 #endif
       feat_ext[feat_ext_count] = NULL;
       qsort((void *)feat_ext, feat_ext_count, sizeof(*feat_ext),