doc: child_process.execFile arguments are optional
doc: describe child_process.fork() silent option
child_process: fix spawn() optional arguments

Spawn's arguments were documented to be optional, as they are for the
other similar child_process APIs, but the code was missing. Result was
`child_process.spawn('node', {})` errored when calling slice() on an
Object, now it behaves as the documentation said it would.