streams: set default hwm properly for Duplex

Default highWaterMark is now set properly when using stream Duplex's
writableObjectMode and readableObjectMode options.

Added condition to the already existing split objectMode test to ensure
the highWaterMark is being set to the correct default value on both the
ReadableState and WritableState for readableObjectMode and
writableObjectMode.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
lib: noisy deprecation of child_process customFds

Use a util.deprecate wrapper to issue warnings like any other
deprecated API. The option has been marked as deprecated in the docs
since v0.5.11.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
doc: remove customFds mentions

The feature has been marked as deprecated since v0.5.11.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
test: Remove references to customFds

Option has been deprecated since v0.5.11

Signed-off-by: Fedor Indutny <fedor@indutny.com>
doc: re-org child_process into async/sync

Cleaner separation between the traditional non-blocking functions and
the new blocking variants.

Signed-off-by: Fedor Indutny <fedor@indutny.com>