Add curl_multi_fdset to PHP cURL extension

Description

The binding for curl_multi_fdset() (available in libcurl C API) is missing.

This creates a limitation:

• In libcurl (C), developers can extract the underlying sockets and integrate them into a single event loop with other file descriptors (e.g. database sockets, custom UDP/TCP sockets, message queues, etc.) using select(), poll(), or epoll().
• In PHP, it is not possible: we only have curl_multi_select(), which is isolated and works exclusively for cURL sockets.

Many PHP applications combine multiple asynchronous sources:

• HTTP requests via cURL multi-handle
• Database connections (PostgreSQL via pg_socket())
• Custom sockets (e.g. UDP for inter-process communication)

Proposed:

curl_multi_fdset(CurlMultiHandle $mh, array &$read, array &$write, array &$except): int

Where:
$read, $write, $except are arrays of PHP stream resources corresponding to the fds returned by libcurl.
Return value is the max_fd (as in libcurl).

Example:

$read = [$pg_socket, $udp_socket];
$write = [];
$except = [];

curl_multi_fdset($mh, $read, $write, $except);
stream_select($read, $write, $except, $timeout);