child_process: fix assertion error in spawnSync

When ExitCallback was not called with an error such as ENOENT in
uv_spawn, the process handle still remains refed and needs to be closed.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
test: update test-dns.js after a60a9b0

resolveTxt now returns a 2-d array for all the chunks string chunks
relating to the record
stream: split `objectMode` for Duplex

This commit introduces `readableObjectMode` and
`writableObjectMode` options for Duplex streams.
This can be used mostly to make parsers and
serializers with Transform streams.

Also the docs section about stream state objects
is removed, because it is not relevant anymore.
The example from the section is remade to show
new options.

fixes #6284

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>