udp_wrap: don't use namespace imports in header

It's not really harmful but it's inconsistent with the other header
files.
udp_wrap: use proper imports in source file
stream_wrap, udp_wrap: add read-only fd property

Expose the file descriptor as a read-only property on the internal
handle objects. Intended for debugging purposes, not part of the API
proper. The property is always null on Windows.

Fixes #4754.