This type hint is a syntax only supported by PHPStan, so it generates errors in other systems such as Intellephense

After updating selfoss to SimplePie 1.9.0, PHPStan started to complain:

    Parameter #1 $level of method SimplePie\SimplePie::set_autodiscovery_level() expects 0|1|2|4|8|16|31, 7 given.

Yet, the PHPDoc claimed:

    level can be a combination of the above constants, see bitwise OR operator

Let’s swtich to `int-mask-of`.

`CURLINFO_REDIRECT_COUNT` is only non-zero when `CURLOPT_FOLLOWLOCATION` is enabled
but we track redirects ourselves since 692e8bc.

https://curl.se/libcurl/c/CURLOPT_FOLLOWLOCATION.html
https://curl.se/libcurl/c/CURLINFO_REDIRECT_COUNT.html

Since starting to handle redirects ourselves in 692e8bc, `url` info item (aka `CURLINFO_EFFECTIVE_URL`) will be the same as the initial URI.

In fact, when the server sends interim responses, this was counterproductive – it would overwrite the final response code returned by `CURLINFO_HTTP_CODE` with the interim one (e.g. 100).
Though, the parser will still include the headers of all but the first response in the body.

Explicit comparison is clearer than relying on `CURLE_OK` being falsey (`0`).

This is only needed for PHPStan.

Making the type assertion explicit and moving it earlier will allow us to make the `prepareHeaders` call conditional without having to add more casts.

The method call was introduced in 4b1fc33 to support tunneling through HTTP proxy using CONNECT method, whose response curl includes in the `curl_exec` result.

Ideally, we would prevent that with `CURLOPT_SUPPRESS_CONNECT_HEADERS` but that is only available in PHP 7.3:
https://www.php.net/manual/en/curl.constants.php#constant.curlopt-suppress-connect-headers

The implementation of magic_quotes_gpc was removed in PHP 5.4 and the function for checking was dropped in PHP 8.0:
https://www.php.net/manual/en/function.get-magic-quotes-gpc.php

This will slightly simplify the code.

It is an ancient technology for using custom fonts based on Flash.
It will not work in any modern browser.

The test runner was removed in 0fcde72.

Multifeed has been deprecated since ff1b513.