wp-admin/post.php > sendback URL generated through string concatenation

[podpirate]

This will result in invalid redirect URLs (containing two ?) when admin_url filter is applied before.
Patch uses add_query_arg() rather than string concatenation.

[podpirate]

use add_query_argfor sendback URL

[wonderboymusic]

In 31633:

When adding post_type to $sendback in wp-admin/post.php, use add_query_arg() instead of string concatenation.

Props podpirate.
Fixes #31492.