Functionality for inheriting FDs

As described here http://mid.gmane.org/20151030-inherit-fds-w-o-sd_listen_fds-in-pure@ruby

```
# see the sd_listen_fds(3) manpage for details
sd_pid, sd_fds = ENV.values_at('LISTEN_PID', 'LISTEN_FDS')
if sd_pid.to_i == $$ # n.b. $$ can never be zero
#3 = SD_LISTEN_FDS_START
inherited = (3...(3 + sd_fds.to_i)).map { |fd| Socket.for_fd(fd) }
end
```