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.
doc: clarify Windows signal sending emulation
gyp: fix `ninja` build on linux

fix #6679
!cluster: debug statements to track worker lifetime
!commentary
!doc: clarify worker.disconnect exit, and suicide meaning
!need to call nextTick and return this in cluster/child_process
!fix uv.h comments
!fixme cluster debug messages