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/spell.c

@@ -7814,7 +7814,7 @@ spell_add_word (
 
   if (idx == 0) {           // use internal wordlist
     if (int_wordlist == NULL) {
-      int_wordlist = vim_tempname('s');
+      int_wordlist = vim_tempname();
       if (int_wordlist == NULL)
         return;
     }