fix unsafe calls to strerror

using strerror in a multi-threaded program is not advised. This patch
adds iop_strerror that has a similar interface to strerror but is thread
safe.

Change-Id: I3d5e9bb7ff09666c76b7af4913c0120f965afe77
Signed-off-by: Saggi Mizrahi <ficoos@gmail.com>