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/var.c

@@ -153,7 +153,7 @@ static ParameterError varfunc(char *c, /* content */
         /* put it in the output */
         if(curlx_dyn_addn(out, enc, elen))
           err = PARAM_NO_MEM;
-        curl_free(enc);
+        curlx_free(enc);
         if(err)
           break;
       }
@@ -173,7 +173,7 @@ static ParameterError varfunc(char *c, /* content */
         else {
           if(curlx_dyn_addn(out, enc, elen))
             err = PARAM_NO_MEM;
-          curl_free(enc);
+          curlx_free(enc);
         }
         if(err)
           break;