test: test sending a handle twice

Added test-cluster-send-handle-twice.js testing to send a handle
twice to the parent process.
child_process: fix sending handle twice

When sending a socket to a child process via IPC pipe,
`child_process.js` picks a raw UV handle from `_handle` property, sends
it, and assigns `null` to the property. Sending the same socket twice
was resulting in a runtime error, since we weren't handling the empty
`_handle` case.

In case of `null` `_handle` we should send just a plain text message
as passed it was passed to `.send()` and ignore the handle, letting
users handle such cases themselves instead of throwing the error at
runtime.

fix #5469
Merge remote-tracking branch 'upstream/v0.10' into v0.10