PosixSequentialFile::Skip(uint64_t n)

Original [issue 173](https://code.google.com/p/leveldb/issues/detail?id=173) created by guido.reina@yahoo.com on 2013-05-29T04:38:44.000Z:

The method PosixSequentialFile::Skip(uint64_t n) uses fseek, which is defined as:

``` c
int fseek(FILE *stream, long offset, int whence);
```

The problem is that "offset" is 4 bytes in 32bit machines.

Wouldn't it be better to use fseeko?

``` c
int fseeko(FILE *stream, off_t offset, int whence);
```

and in the Makefile have:
-D_FILE_OFFSET_BITS=64