lseek wraps around!

[JustinCappos]

The implementation of lseek has a bug where the data type (i32) isn't large enough and so it will wrap around on large files.

See 678ffb2#diff-97744cd8def1974ce0c338f5dedaa20ce129fd3b1a07b75cddff8eeff3e96b0eR75-R76 for an example.

This should be changed to i64, which will not overflow in practice, at least with current storage sizes. Note that Rust's std::io implementation uses an u64. POSIX's type is system dependent (because, of course it is), but is usually a i64. Note that POSIX also will return -1 and set errno to EOVERFLOW if you go past the end and would wrap around.