src: add default visibility to NODE_MODULE

It's currently not really possible to compile native add-ons with
-fvisibility=hidden because that also hides the struct containing
the module definition.

The NODE_MODULE() and NODE_MODULE_DECL() macros are structured in
a way that makes it impossible to add a visibility attribute manually
so there is no escape hatch there.

That's why this commit adds an explicit visibility attribute to
the module definition.  It doesn't help with node.js releases that
are already out there but at least it improves the situation going
forward.
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