Improve error messages for failed TCP/IP connections without ext-sockets

[clue]

This changeset improves error messages for failed TCP/IP connections when ext-sockets is not available on Linux. This means it will now consistently report errno/errstr on most supported platforms such as this:

$ telnet 192.168.0.1 8080
Trying 192.168.0.1...
telnet: Unable to connect to remote host: Network is unreachable
$ php examples/11-http-client.php 192.168.0.1
RuntimeException: Connection to tcp://192.168.0.1:80 failed: Network is unreachable in …/TcpConnector.php:115

Builds on top of #265, #171, reactphp/dns#171, reactphp/dns#172 and others

[clue]

I've updated this to use the above work-around only when applicable. This is only known to work on Linux and known to not work on Mac and Windows. Using ext-sockets to access the SO_ERROR remains the preferred way, so most other platforms should be supported as well.

For future reference: This work-around uses "error slippage" which has been known to work for decades, see e.g. https://cr.yp.to/docs/connect.html. The recommended way would be to use a read(fd, 0) call, but PHP's fread($stream, 0) rejects empty reads and a fread($stream, 1) will return false without exposing the errno/errstr. Using fwrite($stream, $data) is known to work on Linux, but we might be able to find other platforms that could potentially also take advantage of this. Given that this is only used when ext-sockets is not available and this changeset already provides value on its own, I would suggest leaving this up for a future PR.

This is ready for review :shipit: