fs: fix handling of `uv_stat_t` fields

`FChown` and `Chown` test that the `uid` and `gid` parameters
they receive are unsigned integers, but `Stat()` and `FStat()`
would return the corresponding fields of `uv_stat_t` as signed
integers. Applications which pass those these values directly
to `Chown` may fail
(e.g. for `nobody` on OS X, who has an `uid` of `-2`, see e.g.
https://github.com/nodejs/node-v0.x-archive/issues/5890).

This patch changes the `Integer::New()` call for `uid` and `gid`
to `Integer::NewFromUnsigned()`.

All other fields are kept as they are, for performance, but
strictly speaking the respective sizes of those
fields aren’t specified, either.

Ref: https://github.com/npm/npm/issues/13918
PR-URL: https://github.com/nodejs/node/pull/8515


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

undo accidental change to other fields of uv_fs_stat