Implement new "learn" action.

There are a few loose ends here.  First, learning actions cause too much
flow revalidation.  Upcoming commits will fix that problem.  The following
additional issues have not yet been addressed:

    * Resource limits: nothing yet limits the maximum number of flows that
      can be learned.  It is possible to exhaust all system memory.

    * Age reporting: there is no way to find out how soon a learned table
      entry is due to be evicted.

To try this action out, here's a recipe for a very simple-minded MAC
learning switch.  It uses a 10-second MAC expiration time to make it easier
to see what's going on:

ovs-vsctl del-controller br0
ovs-ofctl del-flows br0
ovs-ofctl add-flow br0 "table=0 actions=learn(table=1, hard_timeout=10, \
	NXM_OF_VLAN_TCI[0..11], NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[], \
	output:NXM_OF_IN_PORT[]), resubmit(,1)"
ovs-ofctl add-flow br0 "table=1 priority=0 actions=flood"

You can then dump the MAC learning table with:

ovs-ofctl dump-flows br0 table=1

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/util.h b/lib/util.h
index 5c8618d3..5ae0775f 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -197,6 +197,9 @@ void ignore(bool x OVS_UNUSED);
 int log_2_floor(uint32_t);
 int ctz(uint32_t);
 
+bool is_all_zeros(const uint8_t *, size_t);
+bool is_all_ones(const uint8_t *, size_t);
+
 #ifdef  __cplusplus
 }
 #endif