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/misc1.c

@@ -3423,7 +3423,7 @@ get_cmd_output (
     return NULL;
 
   /* get a name for the temp file */
-  if ((tempname = vim_tempname('o')) == NULL) {
+  if ((tempname = vim_tempname()) == NULL) {
     EMSG(_(e_notmp));
     return NULL;
   }