Add $offset param to uv_fs_read() to allow seeking on open handles

This PR modifies `uv_fs_read()` with a new `long $offset` parameter. This allows the same file handle to be reused for multiple reads. Previously the only way to access a section of a file that had already been read was to open a new handle.

**Old:**

```c
uv_fs_read(resoruce $loop, zval $fd, long $length, callable $callback)
```

**New:**

```c
uv_fs_read(resoruce $loop, zval $fd, long $offest, long $length, callable $callback)
```

This change represents a minor BC break for existing code using `uv_fs_read()`.