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/hardcopy.c

@@ -2346,7 +2346,7 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
 
   /* If the user didn't specify a file name, use a temp file. */
   if (psettings->outfile == NULL) {
-    prt_ps_file_name = vim_tempname('p');
+    prt_ps_file_name = vim_tempname();
     if (prt_ps_file_name == NULL) {
       EMSG(_(e_notmp));
       return FAIL;