spawn() stdio api

[piscisaureus]

• child_process.spawn() takes stdinStream, stdoutStream, stderrStream as options. This api is not really for public consumption but we should not make it that misleading. In fact it doesn't take a stream, it takes a wrap (only a pipe wrap actually works I think).
• The customFds api mostly got removed, we only accept -1,0,1,2 as file descriptors now so the child can still access the tty. We should simplify this by letting the user set the inheritStdin etc. option so the child process just inherits whatever stdio the parent is using.

[piscisaureus]

@isaacs: I don't know why this was closed. Reopening.

[isaacs]

@piscisaureus Yeah, looks like I closed it... I don't remember doing that. I blame gremlins.