Create less IO errors to improve efficiency of require (forwardport of #8189)

In some environment (especially when coffescript is involved) module.js
will spend a lot of time looking up nonexisting files. This is caused by the
lengthy error creation. Those errors are then immediately caught.

This change makes the lookup much faster, by avoiding the unnecessary error
creation.

module::statPath now uses the new second parameter of fs.statSync function of the
fs.js. There is a similar change for the fs.stat, adding the third parameter.

This is the forwardport of https://github.com/joyent/node/pull/8189
Few things.

1. `throw_safe`, not `throwSafe`.
2. We're using C++, use `bool throw_safe`.
3. I don't want to wrap the `uv_fs_t`. Instead simplify this to the following and attach it to `req->data`:
```cpp
struct node_fs_t {
  bool throw_safe;
  void* data;
}
```
To reduce the number of allocations, keep a pointer to unused `node_fs_t` and make a singly linked list that you can pull from when one is needed.

@bnoordhuis feedback on that approach?