Improved commments.

Force iperf and rpctest to come from local directory (rather than
hard-coded to /root).
Major word-smithing this time.
Added "unset CDPATH" to abspath calculation; avoids problem for
CDPATH users.
Added smarter logic for detecting disks.
Changed so as not to use SUDO unnecessarily.

Added logic to fix abspath calculations when CDPATH is set in the
environment.