ovs-ofctl: Avoid printing false differences on "ovs-ofctl diff-flows".

It is possible for "struct ofpact"s to differ bytewise even if they are
equivalent when converted to another representation, such as OpenFlow 1.0
action format or a string representation.  This can cause "ovs-ofctl
diff-flows" to print surprising false "differences", e.g. as in the bug
report:

    - actions=resubmit(,1)
    + actions=resubmit(,1)

This commit fixes the problem by comparing not just the ofpacts but also
the string representation and printing a difference only if both differ.

Bug #8899.
Reported-by: Luca Giraudo <lgiraudo@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>