nfs-utils: nfs-iostat.py autofs cleanup and option to sort by ops/s

Introduce optparse for managing command usage/help and the statistics
options. This change helps more cleanly add new options such as --sort
while preserving the iostat-like interval, count, and mount point
positional arguments.

Signed-off-by: Lans Carstensen <Lans.Carstensen@dreamworks.com>
Signed-off-by: Steve Dickson <steved@redhat.com>