libnative/io: pwrite() is broken.

[nodakai]

rust/src/libnative/io/file_unix.rs

Line 100 in 5660db2

fn pwrite(&mut self, buf: &[u8], offset: u64) -> IoResult<()> {

It is normal for pwrite() to return a non-zero value (number of bytes written) but mkerr_libc() always turns it into an Err.

[alexcrichton]

These implementations have been removed in the interim.