Refactor vim_tempname

- temp_count is uint32_t now instead of long because it supposed to be
  at most 999999999 (comment on line 5227) temporary files. The most
  probably it was a long for compatibility with systems where int is
  16-bit.
- Use "nvim" as prefix for temp folder name instead of "v"
- Remove unused parameter from vim_tempname

src/nvim/memline.c

@@ -490,7 +490,7 @@ void ml_open_file(buf_T *buf)
 
   /* For a spell buffer use a temp file name. */
   if (buf->b_spell) {
-    fname = vim_tempname('s');
+    fname = vim_tempname();
     if (fname != NULL)
       (void)mf_open_file(mfp, fname);           /* consumes fname! */
     buf->b_may_swap = FALSE;