Add stream socket keepalive context options

[bukka]

This adds so_keepalive, tcp_keepidle, tcp_keepintvl and tcp_keepcnt stream socket context options that are used to set their upper case C macro variants by setsockopt function.

The test requires sockets extension and just tests that the values are being set. This is because a real test would be slow and difficult to show that those options really work due to how they work internally.

[lucasnetau]

Is it possible please to also have TCP_USER_TIMEOUT constant added to sockets and made available to be set in the context along with this change? Helps in detecting half closed connections and goes hand in hand with KEEPALIVE

https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/

This is how I currently need to do it, being able to provide it via context would be a big benefit (For example I need to hack around in ReactPHP/Socket to be able to set this flag because the stream is not easily accessible but context can be provided)

if (!defined('TCP_USER_TIMEOUT')) {
     define("TCP_USER_TIMEOUT", 18);
}
$internalSocket = socket_import_stream($stream);
socket_setopt($internalSocket, SOL_TCP, TCP_USER_TIMEOUT, 15000); //ms

[bukka]

Yeah that could be added but probs in different PR.

[bukka]

I should replace those with php_stream_is

[lucasnetau]

@bukka TCP_USER_TIMEOUT socket option is now available in master since 46e55dd if you would please consider adding a tcp_user_timeout stream socket context option to this PR.

[bukka]

@lucasnetau I have been looking to TCP_USER_TIMEOUT and I can see how it can be useful. However, as far as I see, there are no similar options in other platforms (Windows, FreeBSD, MacOS) so I don't think we should directly expose it in streams. I would say that using socket_import_stream might be better here as one can check if the TCP_USER_TIMEOUT is available and then have the right expectation. Generally I think that the platform specific options should go only to socket extension.