curl_formdata: fix to pass long where missing, document CURLFORM_NAMELENGTH

- lib650: pass `long` to `CURLFORM_NAMELENGTH` in test.
  Spotted by Copilot.
  https://github.com/curl/curl/pull/22011#discussion_r3412407235
  Follow-up to 3620e569b3

- lib650: drop an interim variable, and interim casts.
  Follow-up to 60776a0515 #2747

- curl_formdata.md: document `CURLFORM_NAMELENGTH` on man page.

- curl_formdata.md: pass `long` to `CURLFORM_BUFFERLENGTH` on man page.

- formdata: pass `long` to `CURLFORM_CONTENTSLENGTH` in comment.

Closes #22017

docs/libcurl/curl_formadd.md

@@ -112,6 +112,12 @@ If you pass a 0 (zero) for this option, libcurl calls strlen() on the contents
 to figure out the size. If you really want to send a zero byte content then
 you must make sure strlen() on the data pointer returns zero.
 
+## CURLFORM_NAMELENGTH
+
+followed by a long giving the length of the name. Pass this option to set
+the length of *CURLFORM_COPYNAME* and *CURLFORM_PTRNAME* strings, if they are
+not null-terminated.
+
 ## CURLFORM_FILECONTENT
 
 followed by a filename, causes that file to be read and its contents used
@@ -278,7 +284,7 @@ int main(void)
                  CURLFORM_COPYNAME, "name",
                  CURLFORM_BUFFER, "data",
                  CURLFORM_BUFFERPTR, record,
-                 CURLFORM_BUFFERLENGTH, sizeof(record),
+                 CURLFORM_BUFFERLENGTH, (long)sizeof(record),
                  CURLFORM_END);
 
     /* no option needed for the end marker */

lib/formdata.c

@@ -184,7 +184,7 @@ static void free_formlist(struct FormInfo *ptr)
  *
  * name/value pair where only the content pointer is remembered:
  * curl_formadd(&post, &last, CURLFORM_COPYNAME, "name",
- *              CURLFORM_PTRCONTENTS, ptr, CURLFORM_CONTENTSLENGTH, 10,
+ *              CURLFORM_PTRCONTENTS, ptr, CURLFORM_CONTENTSLENGTH, 10L,
  *              CURLFORM_END);
  * (if CURLFORM_CONTENTSLENGTH is missing strlen () is used)
  *

tests/libtest/lib650.c

@@ -48,7 +48,6 @@ static CURLcode test_lib650(const char *URL)
   struct curl_forms formarray[3];
   size_t formlength = 0;
   char flbuf[32];
-  long contentlength = 0;
 
   static const char testname[] = "fieldname";
   static char testdata[] = "this is what we post to the silly web server";
@@ -83,19 +82,17 @@ static CURLcode test_lib650(const char *URL)
     goto test_cleanup;
   }
 
-  contentlength = (long)(strlen(testdata) - 1);
-
   /* Use a form array for the non-copy test. */
   formarray[0].option = CURLFORM_PTRCONTENTS;
   formarray[0].value = testdata;
   formarray[1].option = CURLFORM_CONTENTSLENGTH;
-  formarray[1].value = (char *)(size_t)contentlength;
+  formarray[1].value = (char *)(strlen(testdata) - 1);
   formarray[2].option = CURLFORM_END;
   formarray[2].value = NULL;
   formrc = curl_formadd(&formpost,
                         &lastptr,
                         CURLFORM_PTRNAME, testname,
-                        CURLFORM_NAMELENGTH, sizeof(testname) - 2,
+                        CURLFORM_NAMELENGTH, (long)(sizeof(testname) - 2),
                         CURLFORM_ARRAY, formarray,
                         CURLFORM_FILENAME, "remotefile.txt",
                         CURLFORM_END);