ofp-print: Print durations with at least three decimals.

ofp-print: Print durations with at least three decimals.

Occasionally I run a command like this:
    watch -n.1 ovs-ofctl dump-flows br0
to see how flows change over time.  Until now, it has been more difficult
than necessary to spot real changes, because flows "jump around" as the
number of decimals printed for duration changes from moment to moment.
That is, you might see
    cookie=0x0, duration=4.566s, table=0, n_packets=0, ...
one moment, and then
    cookie=0x0, duration=4.8s, table=0, n_packets=0, ...
the next moment.  Shortening 4.8 to 4.800 shifts everything following it
two places to the left, creating a visual jump.

This commit avoids that problem by always printing at least three decimals
if we print any.  There can still be an occasional jump if a duration is
exactly on a second boundary, but that only happens 1/1000 of the time.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>