unix, windows: fix uv_fs_chown() function prototype

Before this commit, uv_fs_chown() and uv_fs_fchown() took the uid and
gid as signed integers which is wrong because uid_t and gid_t are
unsigned on most all platforms and IDs that don't fit in a signed
integer do exist.

This is not an ABI change because the size of the uid and gid arguments
do not change, only their sign.

On Windows, uv_uid_t and uv_gid_t are typedef'd as unsigned char for
reasons that are unclear. It doesn't matter: they get cast to ints when
used as function arguments. The arguments themselves are unused.

Partial fix for joyent/node#5890.