netdev-dummy: Remove FreeBSD dependency.

netdev-dummy: Remove FreeBSD dependency.

There's no particular reason that netdev_dummy_register() has to care about
the particular OS, except that the tests like to use the special Linux-only
tunnel vport types.  But that can be done better, I think, by just always
registering them from netdev_dummy_register() and making that function
idempotent, so that calling it twice under Linux has no additional effect.
This commit implements that solution.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ed Maste <emaste@freebsd.org>