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/quickfix.c

@@ -2537,7 +2537,7 @@ static char_u *get_mef_name(void)
   static int off = 0;
 
   if (*p_mef == NUL) {
-    name = vim_tempname('e');
+    name = vim_tempname();
     if (name == NULL)
       EMSG(_(e_notmp));
     return name;