tests/fileio.rc: library for file descriptor based IO in tests

There are situations in test scripts where we want to keep open file
descriptors while performing other commands. Bash has abilities
to manage file descriptors by numbers, but the syntax is a little
brain damaging.

This library provides wrappers around it to abstract away bash's
syntax and also provides a helper function to pick a free file
descriptor on the fly.

The APIs are pretty self explanatory.

Change-Id: I82f1d1957646dd6c468d9e85c90ec30c978c7ad6
BUG: 764966
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4565
Reviewed-by: Amar Tumballi <amarts@redhat.com>