Extract extra info from headers_sent

[undercloud]

headers_sent allows you to define a file and a line that has undesirable output, otherwise it is impossible to identify and correct the error, there is not enough of this and it is difficult to debug, but the edit is very small, but gives many possibilities, I hope this edit will fall into the main branch, thanks!

https://www.php.net/manual/en/function.headers-sent.php