v8's `messages.js` file's `CallSiteGetMethodName` is running through all
object properties and getter to figure out method name of function that
appears in stack trace. This run-through will also read `fd` property of
`UDPWrap` instance's javascript object, making `UNWRAP()` fail.

As a simple alternative to the test case above, one could just keep
reference to the dgram handle and try accessing `handle.fd` after it has
been fully closed.

fix #6536

`Socket` no longer accepts `type` option, and also accepts `readable`,
`writable` options.

fix #6541

Add NOTE section in documentation, mentioning that `randomBytes` won't
block when entropy sources are drained.

fix #6372

Conflicts:
	lib/events.js
	src/udp_wrap.cc