lib/flow: Skip minimask value checks.

We allow zero 'values' in a miniflow for it to have the same map
as the corresponding minimask.  Minimasks themselves never have
zero data values, though.  Document this and optimize the code
accordingly.

v2:
- Made miniflow_get_map_in_range() to return data offset instead of
  a pointer via the last parameter.
- Simplified minimatch_hash_in_range() by removing pointer arithmetic.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
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>