Feature: Support listening on existing file descriptors (FDs) with SocketServer. (#269 by @clue)

$socket = new React\Socket\SocketSever('php://fd/3');

This is particularly useful when using systemd socket activation like this:

$ systemd-socket-activate -l 8000 php examples/03-http-server.php php://fd/3

Feature: Improve error messages for failed connection attempts with errno and errstr. (#265, #266, #267, #270 and #271 by @clue and #268 by @SimonFrings)

All error messages now always include the appropriate errno and errstr to give more details about the error reason when available. Along with these error details exposed by the underlying system functions, it will also include the appropriate error constant name (such as ECONNREFUSED) when available. Accordingly, failed TCP/IP connections will now report the actual underlying error condition instead of a generic "Connection refused" error. Higher-level error messages will now consistently report the connection URI scheme and hostname used in all error messages.

For most common use cases this means that simply reporting the Exception message should give the most relevant details for any connection issues:

$connector = new React\Socket\Connector();
$connector->connect($uri)->then(function (React\Socket\ConnectionInterface $conn) {
    // …
}, function (Exception $e) {
    echo 'Error:' . $e->getMessage() . PHP_EOL;
});

Improve test suite, test against PHP 8.1 release. (#274 by @SimonFrings)