Support listening on existing file descriptors (FDs) with SocketServer

[clue]

This changeset adds support for listening on existing file descriptors (FDs) with the SocketServer:

$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

See also https://www.freedesktop.org/software/systemd/man/systemd.socket.html and http://0pointer.de/blog/projects/socket-activation.html for details on systemd socket activation.

The implementation and test suite is pretty straight forward and somewhat similar to the existing TcpServer and UnixServer except for the constructor using fopen('php://fd/3', 'r') to duplicate an existing file descriptor instead of creating a new socket server. Most of the work went into error reporting across different platforms and properly validating the given file descriptor with the limited low-level functions provided by PHP (which applies some private logic from https://github.com/clue/fd with permission). The test suite has 100% code coverage and should work on all platforms that support file descriptors and access to /dev/fd (which includes Linux and Mac and excludes Windows).

Builds on top of error reporting introduced via #266 and #267
Resolves / closes #164