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/if_cscope.c

@@ -1044,7 +1044,7 @@ static int cs_find_common(char *opt, char *pat, int forceit, int verbose, int us
   if (qfpos != NULL && *qfpos != '0' && totmatches > 0) {
     /* fill error list */
     FILE        *f;
-    char_u      *tmp = vim_tempname('c');
+    char_u      *tmp = vim_tempname();
     qf_info_T   *qi = NULL;
     win_T       *wp = NULL;