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

Adds --sort option to display mount point stats sorted by ops/s
Adds --list=<n> option to only display stats for first <n> mount points
E.g. the use of "--sort --list=1" should be useful in seeing stats for
only the mountpoint with the highest ops/s.

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