Improved error reporting in xio

How portable is strerror? Assuming it is... This patch provides more robust
failure mode reporting in xio.

Pre patch:
curvetun[15819]: ioctl screwed up!

Post patch:
curvetun[15819]: ioctl screwed up! Operation not permitted.
Merge branch 'master' of https://github.com/electronjoe/netsniff-ng