Fixes in pread()

The bug occurs when pread() doesn't return 0 when asked to read an
offset beyond its buffer.

This behavior is explicitly documented at:
http://pubs.opengroup.org/onlinepubs/000095399/functions/read.html

> If the starting position is at or after the end-of-file, 0
> shall be returned
Added to the test_files test to ensure the pread() functionality.
Removed old test case and put a test case in a more appropriate area.

tests/unistd/io.*

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Revert "Added to the test_files test to ensure the pread() functionality."

This reverts commit 2ed2fb5df392251a0957543b920272b9b2ae7908.
Merge pull request #1628 from michaeljbishop/pread-fix2

Fixes in pread()