Improve error messages when accepting connections with errno/errstr

[clue]

This changeset improves the error messages for failed incoming connections to include the appropriate errno/errstr. Reporting the errstr works on all supported platforms, the errno is only available when ext-sockets is available. Whereas it would previously report a generic "Error accepting new connection" error, it will now report the actual underlying error condition, such as this:

-Error accepting new connection
+Failed to accept new connection: Connection timed out

Builds on top of #266, #265, #171, reactphp/dns#171, reactphp/dns#172 and others
Together with #244 this is a prerequisite for #164