ext/sockets: adding Linux's TCP_USER_TIMEOUT constant.

[lucasnetau]

Useful to ensure PHP sockets are notified of socket failure when Sockets may be closed incorrectly and data can no longer be transmitted. eg Firewall dropping connection or peer device failure (commonly seen with Cloud load balancers).

From tcp(7) :It specifies the maximum amount of time in milliseconds that transmitted data may remain unacknowledged, or buffered data may remain untransmitted (due to zero window size) before TCP will forcibly close the corresponding connection and return ETIMEDOUT to the application.

Following #13816 for PR format for socket constants/

[devnexen]

thanks, however it might not be as simple as you thought, the man page says it takes an unsigned int, for the setting part, so you got to makes sure the zend_long >= 0 and <= UINT_MAX. I ll have a look if you can manage the change.