dpif: Support working around actions that a datapath does not support.

dpif: Support working around actions that a datapath does not support.

Until now, OVS has expected that the datapath supports all the actions
required by any flow to be installed.  There are at least two reasons why
a datapath might not support a given action:

    - The datapath version is older than the userspace version, and the
      action was introduced after the version of the datapath in use.

    - The action is not considered important enough to implement as part of
      an ABI that must be maintained forever.

This commit adds infrastructure to handle these cases.  It doesn't actually
add any uses; that will come in an upcoming commit.

Signed-off-by: Ben Pfaff <blp@nicira.com>