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

Update list of mount points at each interval and check for differences
when producing comparative stats.  This ensures proper stats collection
for autofs mountpoints.

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