dummy: Introduce new --enable-dummy=system option.

dummy: Introduce new --enable-dummy=system option.

Until now there have been two variants for --enable-dummy:

    * --enable-dummy: This adds support for "dummy" dpif and netdev.

    * --enable-dummy=override: In addition, this replaces *every* existing
      dpif and netdev by the dummy type.

The latter is useful for testing but it defeats the possibility of using
the userspace native tunneling implementation (because all the tunnel
netdevs get replaced by dummy netdevs).  Thus, this commit adds a third
variant:

    * --enable-dummy=system: This replaces the "system" dpif and netdev
      by dummies but leaves the others untouched.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>