setcookie() minor BC break - fixes issue #67736

Hi,

This is a minor BC break, but still a BC break, so worth discussing on this
ML.

When a second setcookie() is done with the same name, a warning is emitted,
because the ietf rfc 6265 says it *should* only send one Set-Cookie header
per name.

This is fine when display_errors is set to off. When it's set to on, the
warning prevents the header from being added because "headers already sent"
(which is the minor BC break, as current PHP just sends 2 Set-Cookie
headers with the same name).

So, should it be merged? What should be done to comply with the ietf rfc
6265?

PR: https://github.com/php/php-src/pull/795/
PHP issue: https://bugs.php.net/bug.php?id=67736

Regards,

*Florian Margaine*